Added a rotation handle to the map object item

The item is now rotated rather than just showing an arrow.

There are still some problems with this:

* Edit polygon tool does not cope with it
* Would feel less weird when objects rotated around their center
* This makes no sense for non-tile objects on isometric maps

Closes #11  (support for rotating objects)
Closes #325 (rotate object image)

Sponsored-by: Ben Wales
Removed the option to show rotation arrows

If they are to be drawn, it should be done by the map object item and
not by each map renderer. For now I'm just removing it since it doesn't
seem that helpful to me anyway when the objects are already rotated
themselves.
Allow objects to be flipped horizontally and vertically

Flipping is supported for tile objects and polygons, and for both
orthogonal maps and isometric maps.

Flipping currently does not take into account object rotation,
so "horizontal" flipping might mean something else for rotated objects.
Also, when flipping groups of objects, they are flipped individually.
These issues can be resolved later.

Closes #58  (flipping for tile objects)
Closes #116 (applying tile offset to tile objects)
Closes #154 (flipping for polygons and polylines)

Sponsored-by: Ben Wales
Apply flip and rotate shortcuts to selected objects

When applicable, the X and Y shortcuts will now flip selected objects
and the Z and Shift+Z shortcuts will rotate them.
Handle object rotation in Edit Polygon Tool