Player: remove unused get_surface method
Player/main: -1 is up and 1 is down

To make it consistent with the pixel calculations. ((0, 0) is
the top left corner, so applying -1 is moving something up)
Represent velocity as vector

Instead of (xy)direction (xy)speed. It makes easy to calculate
new trajectory in collision.
Collision: Add getH and getV to Clutter.Rectangle prototype.

They are really operations of the objects.
Move also segment to the collision engine (only user so far).
Update collision unit tests to new calculations

We now support overlaps not only exact matches so some previous assertions
were wrong.

Also objects now must have getV and getH and ball must have velocity