[FEATURE] Brush stabilizer patch by Juan Luis Boya García

This commit adds to calligra a new stabilizer feature as an alternative
to the current "Weigthed smoothing".

This stabilizer is very simple, calculating an arithmetic mean of the
last N values (position, pressure and tilt) each millisecond and drawing
a line from the previous point to there.

This takes into account both the length of the strokes and the speed
they are made with.

Fast and small movements are considered erratic and are likely to be
ignored yielding a mostly straight line.

On the other hand, slower and bigger movements are assumed to be
deliberated and will follow the shape the user draws. The slower and
bigger, the more accurate.

This process makes the strokes draw 'slowly'. The user sees how the line
pursues their cursor. The effect is harder or softer depending on the N
chosen (sample size), which is user-customizable.

Starting and end points are forced by the algorithm to be drawn exactly
where the user began and ended the line.