Basic implementation of tracking based on center

- capability to add center by right click
- if center is defined it is used in tracker initialization
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)
fixed flags for QGraphics(Spot/Center)Item
made size of center settable in config
better uncertainty estimation for initial velocity
changed default parameters for input precision and process noise
Merge branch 'develop'

implemented (optional) user input of the center for more reliable
tracking