Added virtual wxPGProperty::ValueToString(). In derived property classes, now it...

Added virtual wxPGProperty::ValueToString(). In derived property classes, now it must be implemented instead of GetValueAsString (assertion failure is raised at run-time if you fail to do so). This change is needed to properly support wxEVT_PG_CHANGING for nested composite string properties. wxPGProperty::GenerateComposedValue() partially updated to support this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56363 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775