Credit: EmPeWe

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.

- 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

    - Qt plugins and libraries (and associated files) bundled only if necessary
    - Dependencies corrected (for Ubuntu 14.04 and 16.04, at least),
    mostly manually for now
    - run.sh sets Qt plugin path environment variable (=> qt.conf
    no longer needed; removed)

- Separated build and package
- OpenBoard now installs in /opt
- All binaries are stripped, for smaller package size

Credit: Jaroslav Ryník

I can't get the mobile site to display correctly; the position of the
contents moves around depending on the widget's position on the board.

Switched back to the desktop site because it's not worth spending days
on this.

Credit: Jaroslav Ryník

also added a function to get slightly larger images (though the Pixabay
API doesn't give width and hight info about these, so it's not currently
used)

- Fixed display
- Fixed/added support for pages
- Added image title generation, so it can be e.g added to OpenBoard
library

removed references to youtube from videoPicker widget (youtube embedding doesn't work, won't be fixed soon)

Search basically works; but just one page of elements are retrieved.
Metadata is assigned ok.
To do: paginate search results; correct display; and later, add filters