Remove the port files and references to it from the common headers and
elsewhere.
Also remove GPE (GNOME PDA Environment) support as libgpewidget is
unmaintained since 2006 or so and has never been really used.
Use __WXGTK__ to test for any version of wxGTK now. Still define
__WXGTK20__ for compatibility, but always define it now and don't test
for it in the library code.
fb4f0b590c