It was unexpected that using wxSizerFlags().Right().CentreVertical()
didn't right-align the item because CentreVertical() reset the alignment
in the horizontal direction rather than just setting it in the vertical
one, so change this, document the new behaviour explicitly and add a new
unit test checking for it.
Closes#18989.
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).
