Add input method support

; The code is basically refactored from
; https://github.com/ch11ng/exim to get better maintenance.

* exwm-xim.el: New module making Emacs's builtin input methods usable
for interacting with X windows.

* exwm-core.el (exwm--intern-atom): New function for intern X11 atoms.
* exwm-input.el (exwm-input--init):
* exwm-manage.el (exwm-manage--init): Use it.

exwm-core.el

@@ -131,6 +131,15 @@ Nil can be passed as placeholder."
                                          (if height xcb:ConfigWindow:Height 0))
                      :x x :y y :width width :height height)))
 
+(defun exwm--intern-atom (atom)
+  "Intern X11 ATOM."
+  (slot-value (xcb:+request-unchecked+reply exwm--connection
+                  (make-instance 'xcb:InternAtom
+                                 :only-if-exists 0
+                                 :name-len (length atom)
+                                 :name atom))
+              'atom))
+
 (defmacro exwm--defer (secs function &rest args)
   "Defer the execution of FUNCTION.