Add support for wxStaticBoxSizer "windowlabel" property to XRC

Allow specifying arbitrary windows as labels for the static boxes
created from XRC.

Note that wxStaticBox XRC handler itself wasn't updated to support this,
as this handler seems to be quite useless because it's impossible to
define any box children with it, so only wxStaticBoxSizer XRC handler
really matters, anyhow.

docs/doxygen/overviews/xrc_format.h

@@ -2416,6 +2416,9 @@ support the following properties:
     Sizer orientation, "wxHORIZONTAL" or "wxVERTICAL" (default: wxHORIZONTAL).}
 @row3col{label, @ref overview_xrcformat_type_text,
     Label to be used for the static box around the sizer (default: empty).}
+@row3col{windowlabel, any window,
+    Window to be used instead of the plain text label (default: none).
+    This property is only available since wxWidgets 3.1.1.}}
 @endTable
 
 @subsection overview_xrcformat_wxgridsizer wxGridSizer