Map Object tile resizing and object group resizing, v2.

After receiving feedback from users and developers, I revamped the last pass at object tile resizing. Summary:
- Map renderer supports scaling of object tiles.
- Map reader and object tile creation will correctly initialize the size of the tile. (Maybe the writer should detect if the tile is default size and omit tile size?)
- Removed resize handle from mapobjectitem.cpp, though I left the generic handle code if it may be needed at another time.
- In objectselectiontool.cpp, added a set of 8 resize handles and modified the Corner enum into AnchorPosition, representing edge centers as well.
- When a single object is selected, 8 resize handles are used, aligned with the object's rotation.
- When multiple objects are selected, 8 resize handles are used but the scaling is locked to aspect ratio.
- Object tile size is stored in correct pixel dimensions, but through an ugly special case.

Caveats:
- Object tiles have their dimensions in pixels, whereas most other objects are in tile size units.
- Isometric renderer does not support at this time. The boundingRect is not correctly updated, so it won't work.
- Still no support for polylines and polygons.
- Negative scaling factors are disallowed.