[bugfix] idle_inhibitor handles click events correctly

- Declare event handler in ALabel virtual so the idle_inhibitor can
  overriding them
- Handle the right click event in idle_inhibitor and call ALabel handler if needed

include/modules/idle_inhibitor.hpp

@@ -13,7 +13,7 @@ class IdleInhibitor: public ALabel {
     ~IdleInhibitor();
     auto update() -> void;
   private:
-    bool onClick(GdkEventButton* const& ev);
+    bool handleToggle(GdkEventButton* const& e);
 
     const Bar& bar_;
     std::string status_;