Add support for event processing via XInput 2

Add support for event processing via XInput 2

* configure.ac: Add an option to use XInput 2 if available
* src/Makefile.in (XINPUT_LIBS, XINPUT_CFLAGS): New variables
(EMACS_CFLAGS): Add Xinput CFLAGS
(LIBES): Add XInput libs
* src/xmenu.c (popup_activated_flag): Expose flag if XInput 2 is
available
* src/xfns.c (x_window): Set XInput 2 event mask
* src/xterm.c (x_detect_focus_change): Handle XInput 2 GenericEvents
(handle_one_xevent): Handle XInput 2 events
(x_term_init): Ask the server for XInput 2 support and set xkb_desc if
available
(x_delete_terminal): Free XKB kb desc if it exists, and free XI2
devices if they exist
(x_free_xi_devices, x_init_master_valuators): New functions
(x_get_scroll_valuator_delta): New function
(init_xterm): Don't tell GTK to only use Core Input when built with
XInput 2 support
* src/xterm.h (struct x_display_info): Add fields for XKB and XI2
support
* src/gtkutil.c (xg_event_is_for_menubar): Handle XIDeviceEvents
(xg_is_menu_window): New function
(xg_event_is_for_scrollbar): Handle XIDeviceEvents