Refactor: extract XRC code handling conversion from dialog units.

Factor out this code to a reusable ParseValueInPixels() function instead of
repeating it in GetSize() and GetDimension() (and using a hack to reuse it
from GetPosition()).

No real changes, just made some error messages more precise.

include/wx/xrc/xmlres.h

@@ -585,6 +585,8 @@ public:
     // Gets the value of a boolean attribute (only "0" and "1" are valid values)
     bool GetBoolAttr(const wxString& attr, bool defaultv);
 
+    // Returns the window associated with the handler (may be NULL).
+    wxWindow* GetParentAsWindow() const { return m_handler->GetParentAsWindow(); }
 
     // Sets common window options.
     void SetupWindow(wxWindow *wnd);