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.