Implement bindsym --release

This is a "simple" version of --release (same as i3) that only supports
a binding that contain one normal key. e.g.:

    bindsym --release $mod+x exec somthing-fun

I didn't bother implementing it for a combination like `$mod+x+z` since
it is a bit tricky to get right and also a bit weird to actually do on a
keyboard.

include/config.h

@@ -22,6 +22,7 @@ struct sway_variable {
  */
 struct sway_binding {
 	int order;
+	bool release;
 	list_t *keys;
 	uint32_t modifiers;
 	char *command;