1. added wxRegionBase class and derive all wxRegions from it
2. also added wxRegionWithCombine for the ports providing Combine() method
(MSW, OS2 and Mac) to reduce code duplication further
3. made sure region comparison works the same way in all ports (except those
not implementing it) and added IsEqual() method
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41429 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
