Add include/dialog_shim.h and common/dialog_shim.cpp for use by wxformbuilder's "subclass a wxDialog" support.
This works, but in my version of wxformbuilder there is a bug which does not properly show the 
subclass property, even though it is still in play.  This happens after saving then loading the *.fbp file.
So it is a nuisance bug, but does not affect functionality.