Add a new GimpData::data_cancel() virtual function, and a
corresponding gimp_data_factory_data_cancel() function.  This
function should cancel any ongoing async operations related to the
factory (i.e., included in its async set), and wait for the
operations to finish.  Provide a default implementation that simply
cancels and waits on the factory's async set.

Use this function to cancel any ongoing operations during factory
destruction, and in gimp_data_factory_data_free().

Override this function in GimpFontFactory, for which we can't
really cancel font loading, and simply cancel and clear the
factory's async set without waiting for loading to finish, making
sure that nothing happens (and, in particular, that the factory
isn't being accessed, since it might be already dead) when loading
does finish.