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).

The scale of PDF items was sometimes badly calculated when opening a
document made with a previous version of OpenBoard or made on another
computer.

Specifically, this solves the following issues:

- PDF scale calculation in documents that did not specify the pageDPI
used to render the PDF (happened with documents created with some old
versions of OpenBoard)

- PDF scale calculation in multi-page documents (it was set correctly
for the first page, but not the following ones)

In some cases, the PDF background of a document could be scaled badly
when tools such as the ruler, compass etc. were present on the page.

This happened with PDFs of version <= 1.4, and when the tools were
outside of / larger than the page.

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.

This adds screencast support on Linux. This is based on
ffmpeg for the encoding & muxing (default format is MP4 with AAC audio
and H264 video).
The microphone sound is grabbed using QAudioInput. The encoder should
be able to handle virtually any input format (it has been tested with
a stereo input at different sampling rates with one format (16-bit
signed) but it should work fine with any number of channels, sample
format etc.).

The only problems I have run into so far are that desktop recording is
very slow (compared to OS X) and that the last few video frames are
usually not included in the video. This may be due to GOPs not being
complete, but that's just a wild guess.

Brazilian portuguese translation

Fix for window being placed badly on the screen when returning to board mode from desktop mode on some environments

Also, clarified the difference between received and drawn points
in UBGraphicsStroke

UBGraphicsScene::drawCurve can be used either way)

- Added a curveToPolygon function that creates a curved polygon,
eliminating the need to generate lots of small ones to make a curve look
smooth.

- Cleaned up the rest of the code a bit

This reverts commit 177571ca79cbd92f6544c80ce36ded88ce64fa7a.

- UBGraphicsScene calls UBGraphicsStroke::addPoint, which returns a list
of points that can be drawn. It may be none (e.g we discard very small
segments), one (if we do no interpolation) or several.

- Added a UBInterpolator base, abstract class. Various interpolation
methods can be added easily.

- Current methods: Basic spline (custom), Catmull-Rom spline (based on
alglib), and Bézier

- Added a setting to toggle interpolation. Added this to the UI as well

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)