cleanup
Changed size of TestCards

The new cards have been made smaller to make them fit into the
SnapAreas, which will soon be implemented ;)
Add CommonConstants

Provides public-static-final-"configurations" needed by different parts
of the program. These should perhaps be configurable by user in the
future.
Changed mouse interaction with cards.

Cards now check with the CommonConstants if they are in a good place
before setting their positions in the model.
cleanup
Removed drag limit.

TestgameBoard does not need to limit the cards to itself since they
replaces themselves back on the board if dropped outside it.