Added selection for actors and simple movement.
Changed collision detection from bounding-boxes to abstract x-y-coordinates + scale.
Added the possibility to add a new unit to the Grid:
- Added first skeleton of a Scene2d UI that contains the placable units.
- Added placement logic to Map and Controllers.
Changed the panning and zooming code to be more user-friendly
and less jagged.

- the pan-code now translates the coordinates into the grid-coordinates
  and scrolls accordingly.
- the zoom-code now only zooms 1/10 as fast as before.