Setting wxFLEX_GROWMODE_ALL for the non-flexible direction broke
handling of proportions in the flexible direction which were
unexpectedly not taken into account at all any more due to not passing
them to DoAdjustForGrowables().
Fix this by still respecting the proportions in this case and add a test
case to ensure this doesn't get broken again.
See #23251, #23253.
(cherry picked from commit 6bfabd7afe16ed734565635228d540cb47b16970)
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).
