group: fix hover regression

We aren't including the hover detection on the revealer, so when the
animation fires we fire the leave event which starts an infinite loop of
enter/leave while we watch boxes move back and forth.

include/group.hpp

@@ -30,6 +30,7 @@ class Group : public AModule {
   bool handleMouseEnter(GdkEventCrossing *const &ev) override;
   bool handleMouseLeave(GdkEventCrossing *const &ev) override;
   bool handleToggle(GdkEventButton *const &ev) override;
+  void addHoverHandlerTo(Gtk::Widget &widget);
   void show_group();
   void hide_group();
 };