naive implementation of 'bindsym --release' (and bindcode)

The implementation is naive because the user has to generate exactly the
event he specified. That is, if you use this binding:

    bindsym --release $mod+x exec import /tmp/latest-screenshot.png

Then it will only be triggered if you hit $mod, hit x, release x,
release $mod. It will not be triggered if you hit $mod, hit x, release
$mod, release x. The reason is that the KeyRelease event in the latter
case will not have the modifier in its flags, so it doesn’t match the
configured binding.

include/config.h

@@ -312,7 +312,7 @@ void switch_mode(const char *new_mode);
  * or NULL if no such binding exists.
  *
  */
-Binding *get_binding(uint16_t modifiers, xcb_keycode_t keycode);
+Binding *get_binding(uint16_t modifiers, bool key_release, xcb_keycode_t keycode);
 
 /**
  * Kills the configerror i3-nagbar process, if any.