Validate wxBoxSizer flags instead of silently ignoring invalid combinations.

Detect using flags corresponding to the major sizer direction (which doesn't
make sense as only the proportion governs the behaviour in this direction) and
also combinations of alignment flags with wxEXPAND.

docs/changes.txt

@@ -24,6 +24,8 @@ Changes in behaviour not resulting in compilation errors
 - Creating wxBitmap with 0 width or height now always fails in all ports
   (it used to succeed in wxMSW).
 
+- Using invalid flags with wxBoxSizer items now triggers asserts.
+
 Changes in behaviour which may result in build errors
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