Revert "due to suspected instability with persistance Worker (randomly not persisted scenes, very hard to reproduce), switched to 'immediateSave mode' (at least for now)"

This reverts commit ae1a2a53.

due to suspected instability with persistance Worker (randomly not persisted scenes, very hard to reproduce), switched to 'immediateSave mode' (at least for now)

fixed a crash where a scene was removed : the compactCache method has no more sense now that we load each scene to create a view on it (thumbnail views)

the last active page

Web results still show the details which include some metadata about the
image retrieved, for example.

This is now a setting so it can easily be re-enabled if desired (which
could be useful especially if we decide to also display metadata of
local items)

We can (again) check for updates and, if an update is available, send
the user to the site to download them.

The old format (a .json specifying a version number and download URL)
was kept. The address for this file is now specified in the settings.

Images (UBGraphicsPixmapitem and UBGraphicsSvgItem) can be set as
background via the menu on their frame. Currently, the image is first
centered and un-transformed before being set as background.

The option in the frame appears for any UBGraphicsItem for which
`data(UBGraphicsItemData::ItemCanBeSetAsBackground)` is true.

This is (currently) only enabled for image items.

We can (again) check for updates and, if an update is available, send
the user to the site to download them.

The old format (a .json specifying a version number and download URL)
was kept. The address for this file is now specified in the settings.

Also install the importer executable to OpenBoard root install folder

This removes a few instances of deleting a scene twice, or accessing
elements of a scene after they've been deleted.

Previously, the application would crash upon exiting if the scene was
empty but had been modified (e.g if an object was placed on the board
then deleted, then the application closed)

This prevents crashes that may occur when deleting multiple pages from a document.

This allows users to change the color of the background grid e.g if they
are using a projector or other low-contrast display.

The settings are in the `Board` category and are named `CrossColorDarkBackground`
and `CrossColorLightBackground`. They take strings representing the
color in any of the following formats:

- #RGB (Hexadecimal digits)
- #RRGGBB
- #AARRGGBB
- #RRRGGGBBB
- #RRRRGGGGBBBB
- Any SVG color keyword name (as defined by W3C)

If enabled in the preferences menu, pen and marker strokes will be
replaced by a simplified stroke after they are drawn.

The algorithm is very basic (for now): if three points are almost lined
up (the threshold angle can be specified in the config file), then the
middle one is removed. This is repeated over the whole stroke; new
polygons are then generated based on the simplified stroke points.

This typically cuts down on number of points and polygons by a factor of
about 10, while having minimal visual impact.

Several issues remain with multi-screen mode on Linux. The behavior is
inconsistent from one desktop evironment to the next, making it hard to
work around these problems. Among the known issues at this stage:

On Ubuntu 14.04, a call to QWidget::setGeometry requires the widget to
be hidden on KDE, but visible on MATE, for the geometry changes to take
effect.
Despite the widget's geometry being updated by this call, the windows
aren't necessarily moved. Meaning that the control and display widgets
will tend to be displayed on the same monitor, even though their
positions are correctly set to different areas on the extended screen.

In the current state, this behavior is observed on MATE. Unity works
fine and KDE only has transient positioning issues (for example,
swapping control and display windows in multi-screen mode leads to both
windows being placed on the same monitor, until multi-screen mode is
turned off then on again).

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Fixed type testing of media items (qgraphicsitem_cast couldn't tell the difference between audio and video items)

- The background selection palette now includes a slider to change the
size of the background grid. Default min/max values are 16 and 64px,
defined in UBSettings. Grid resizes dynamically as the slider is moved.
- Measuring tools' (ruler, triangle) markers follow grid size: 1 square
of the background grid corresponds to 1cm
- Grid size can be different for each page of a document
- Grid size is saved in the .svg
- Documents with a background grid but no specified grid size follow the
default size defined in UBSettings.

Previously, grid size was calculated based on DPI, which can vary from
one OS, computer or display to the next. This new setting allows
documents to be migrated from one machine to another with no unexpected
changes in grid size happening. It also makes it easy to correct any
problems importing old documents (whose grid size might be smaller or
larger than expected when imported on a new version of OpenBoard).

This fixes an issue where if one document was imported with a different
DPI than the current one, any document created thereafter would have
this same value (which could then cause problems if a PDF was added to
that new document).

Saving this value to UBDocumentProxy not only makes more sense, it also
fixes this issue.