Add "checked" property for toolbar tool elements in XRC.

Allow toolbar tools to be created in checked (or toggled) state in XRC, just
as the menu items can already be created checked.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69487 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/xrc_format.h

@@ -1654,6 +1654,8 @@ properties:
     Help text shown in statusbar when the mouse is on the tool (default: none).}
 @row3col{disabled, @ref overview_xrcformat_type_bool,
      Is the tool initially disabled (default: 0)?}
+@row3col{checked, @ref overview_xrcformat_type_bool,
+     Is the tool initially checked (default: 0)? (only available since wxWidgets 2.9.3)}
 @endTable
 
 The presence of a @c dropdown property indicates that the tool is of type