group: disconnect pending reveal timeout in destructor to fix UAF

With reveal-delay set, handleMouseEnter arms a Glib::signal_timeout that
captures 'this'. sigc::connection's destructor does not remove the GLib
source, so a Group destroyed with a pending reveal timer would fire the
timeout on freed memory. Add a destructor that disconnects reveal_timeout_.

include/group.hpp

@@ -14,7 +14,7 @@ class Group : public AModule {
 
  public:
   Group(const std::string&, const std::string&, const Json::Value&, bool);
-  ~Group() override = default;
+  ~Group() override;
   auto update() -> void override;
   operator Gtk::Widget&() override;