the last active page

fixed a bug where thumbnail view was not centering on the active thumbnail view, when clicking on the Next Icon (main bar)

Revert "workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes"

This reverts commit 0fd15f62.

workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes

Removed 'Set as background' button for images, since that feature is now accessible from the image menu on the board

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)

This fixes two related issues:
1) When taking a partial screenshot of the desktop, then copying it
(Ctrl-C) and pasting it in a new document, it was not saved
2) When a page of one document was copied into another (in document
mode, by dragging the page onto another document), media files
disappeared from the new page.

When drag-n-dropping a document to the trash, it would reappear in the
"untitled documents" folder on the next restart. (This was introduced by
bd3d8e95)

In the right-hand pane, two folders that were at the same path and whose
names started with the same characters were considered to be nested by
the breadcrumbs trail.

E.g, folders named "abc" and "abcd", both in the "Audio" folder:
clicking on "abcd" made the breadcrumb trail display "[Audio] > [abc] >
[abcd]"

- 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).

Caused problems e.g with podcast mode, where if the control and display
views were swapped in the preferences, the wrong screen would be recorded
when switching to desktop mode during recording of the podcast.
This happened even if only one screen was plugged in, so a black screen
was recorded in that case (at least on OS X 10.10)

Also quick code clean-up in UBDocumentController