Drawing of set of boxes, union of boxes and lines + various improvements

- drawBoxes: draw a set of boxes from a vector of boxes (vector of
bounds)
- drawBoxesUnion: same input as drawBoxes, but computes the union of
boxes before displaying
- drawLine: draw a line from a vector of points (vector of coordinates),
of from two vectors of x an y coordinates. Fixes #12
- Moved VibesGraphicsItem factory from VibesScene2D to a static
VibesGraphicsItem function.
- Modified cast functions for VibesGraphicsItem* <-> QGraphicsItem*