Bump minimum GTK versions to 2.24 and 3.10

Bump minimum GTK versions to 2.24 and 3.10

* configure.ac: Bump required GTK 2 and GTK 3 versions and the
associated GLib versions. Remove obsolete AC_CHECK_FUNCS calls. These
check for functions available in later GTK 2 versions. These checks
and can safely be removed with the exception of
gtk_window_set_has_resize_grip, which according to a comment in
gtkutil.c causes an issue in Ubuntu's GTK 2.

* src/gtkutil.c:
* src/xfns.c:
* src/xterm.c:
* src/xterm.h: Remove now unused conditional blocks. Use HAVE_GTK3
instead of GTK_CHECK_VERSION where now applicable. Remove checks of
now always true USE_GTK_TOOLTIP.