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.

include/wx/sizer.h

@@ -941,6 +941,9 @@ public:
     virtual void RecalcSizes() wxOVERRIDE;
 
 protected:
+    // Only overridden to perform extra debugging checks.
+    virtual wxSizerItem *DoInsert(size_t index, wxSizerItem *item) wxOVERRIDE;
+
     // helpers for our code: this returns the component of the given wxSize in
     // the direction of the sizer and in the other direction, respectively
     int GetSizeInMajorDir(const wxSize& sz) const