Map Object resizing, v3!

Depends on pull requests #591 and #592, so you might want to get through them first to make the comparison less unsightly. Replaces pull #485. Does the following:

- Map renderer supports scaling of cells.
- Map reader and object tile creation will initialize the size of the object to the tile's size.
- Removed handles from mapobjectitem.cpp.
- In objectselectiontool.cpp, added a set of 8 ResizeHandles, and changed the Corner enum to AnchorPosition, representing the edge centers as well.
- When a single object is selected, resizing is handled in object space.
- When multiple objects are selected, resizing is handled in world space.

Caveats:
- Negative scaling factors are not allowed.