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.

src/handlers.c

@@ -1045,6 +1045,7 @@ void handle_event(int type, xcb_generic_event_t *event) {
 
     switch (type) {
         case XCB_KEY_PRESS:
+        case XCB_KEY_RELEASE:
             handle_key_press((xcb_key_press_event_t*)event);
             break;