Basic implementation of tracking based on center

- capability to add center by right click
- if center is defined it is used in tracker initialization

Signed-off-by: Nicola Ferralis <feranick@hotmail.com>
renaming to more sensible class names

The spot and center positions are displayed on the screen by
classes derived from QGraphicsItems.
They are thus more sensibly named QGraphics*Item (instead of
QGraphics*View)

Signed-off-by: Nicola Ferralis <feranick@hotmail.com>