Complain if wxEXPAND is combined with both horizontal and vertical alignment
flags, as this doesn't make sense and, moreover, the behaviour has changed
since the last commit: now wxEXPAND is ignored instead of overriding the
alignment flags if they are combined.
Allow overriding wxEXPAND effect in one of the directions by specifying
wxALIGN_{RIGHT,BOTTOM} or wxALIGN_CENTRE_{HORIZONTAL,VERTICAL} together with
it (unfortunately this doesn't work for wxALIGN_{LEFT,TOP} as their value is 0
and so their presence in flags can't be detected).
