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.

xinitrc

@@ -1,17 +1,20 @@
-# Disable access control
+# Disable access control for the current user.
 xhost +SI:localuser:$USER
 
 # Make Java applications aware this is a non-reparenting window manager.
 export _JAVA_AWT_WM_NONREPARENTING=1
 
-# Themes, etc
-gnome-settings-daemon &
-
-# Fallback cursor
+# Set default cursor.
 xsetroot -cursor_name left_ptr
 
-# Keyboard repeat rate
+# Set keyboard repeat rate.
 xset r rate 200 60
 
-# Start Emacs
-exec dbus-launch --exit-with-session emacs
+# Uncomment the following block to use the exwm-xim module.
+#export XMODIFIERS=@im=exwm-xim
+#export GTK_IM_MODULE=xim
+#export QT_IM_MODULE=xim
+#export CLUTTER_IM_MODULE=xim
+
+# Finally start Emacs
+exec emacs