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.

Due to antialiasing, adjacent polygons are separated by a very fine
space. The previous solution attempted to hide this by adding a border
to the polygons. The border of adjacent polygons would overlap, which
was visible (despite the attempted color correction) and, more
importantly, caused massive lags especially on Linux.

Therefore it has been removed but feel free to revert this commit some
day and try to fix this more cleanly.

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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#	modified:   src/core/UBDisplayManager.cpp
#	modified:   src/core/UBDisplayManager.h
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text items being editable on page load

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]"

On OS X, making annotations in desktop mode then switching to board
mode and back could cause shadows to be drawn around every stroke, and
these persisted after erasing the strokes (though they did disappear
upon switching to board mode and back again).

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

Adds (experimental) stroke simplification methods

- 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).