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.

docs/doxygen/overviews/xrc_format.h

@@ -1900,6 +1900,9 @@ No additional properties.
     Initial value of the control (default: empty).}
 @row3col{maxlength, integer,
     Maximum length of the text which can be entered by user (default: unlimited).}
+@row3col{forceupper, @ref overview_xrcformat_type_bool,
+    If true, use wxTextEntry::ForceUpper() to force the control contents to be
+    upper case.}
 @row3col{hint, @ref overview_xrcformat_type_text,
     Hint shown in empty control (new since wxWidgets 3.0.1).}
 @endTable