QFlags: use unsigned int when the enum's underlying is unsigned

QFlags: use unsigned int when the enum's underlying is unsigned

Replace storage and operator int() return types with unsigned int
if the enum is unsigned.

This fixes a number of exisiting warnings, in particular with
Qt::MouseButton under GCC.

Change-Id: Ia12d36212329aec3e9d62a5fbd38809a6c2b36d0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>