Credit: ciampix

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)