Change Rect to Rect2d in Tracker::update() and ::init()

I've changed Rect to Rect2d in Tracker::update(), Tracker::init() and
all related methods (including documentation). This allows to initialize
trackers with double-valued rectangles, thus adding versality. Besides,
trackers also can output double-valued rectangles, which may be
beneficial in some scenarios.

However, it remains to change UML diagrams in documentation to tracker
module, as they still display methods above with old signatures.