Add autoconf and bakefile support for detecting and using GTK+4, similar
to how GTK+3 is detected and used.
GTK+3 is checked for first before GTK+4, this can be overridden in
`./configure` using `--with-gtk=4`.
With GTK+4 the `__WXGTK4__` `#define` is also set in `setup.h.in`.
This commit does not regenerate any files.