Implement new 'sticky' command to manually set, remove or toggle the sticky state on a window.

include/commands.h

@@ -204,6 +204,12 @@ void cmd_focus(I3_CMD);
  */
 void cmd_fullscreen(I3_CMD, char *action, char *fullscreen_mode);
 
+/**
+ * Implementation of 'sticky enable|disable|toggle'.
+ *
+ */
+void cmd_sticky(I3_CMD, char *action);
+
 /**
  * Implementation of 'move <direction> [<pixels> [px]]'.
  *