Add support for "forceupper" attribute to wxTextCtrl in XRC

Call ForceUpper() if this attribute is specified.

Currently this is done only for wxTextCtrl but could be extended to wxComboBox
later too if necessary.

misc/schema/xrc_schema.rnc

@@ -1432,6 +1432,7 @@ wxTextCtrl =
         stdWindowProperties &
         [xrc:p="o"] element value     {_, t_text }* &
         [xrc:p="o"] element maxlength {_, t_integer }* &
+        [xrc:p="o"] element forceupper{_, t_bool }* &
         [xrc:p="o"] element hint      {_, t_text }*
     }