Inherit wxURL from wxURI, providing assignment, copy construction, comparison, and...

Inherit wxURL from wxURI, providing assignment, copy construction, comparison, and less duplication of code.  Change wxURI a bit to meet some of Vadim's reccommendations - move accessors into header, and finish some of his other reccom.  Change assignment to use const wxString& instead of const wxChar*.  Change wxURI docs to reflect that it inherits from wxObject.  Made preliminary docs for the wxURL transition.  Add some unit tests for the transition.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30136 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775