Wire in the first public widget signal handler.

These signals should replace the callbacks, but this project will be
done post 1.8. But it shows the design idea.

src/gui/widgets/button.cpp

@@ -43,6 +43,18 @@ tbutton::tbutton()
 				&tbutton::signal_handler_left_button_click, this, _2, _3));
 }
 
+void tbutton::connect_signal_mouse_left_click(
+		const event::tsignal_function& signal)
+{
+	connect_signal<event::LEFT_BUTTON_CLICK>( signal);
+}
+
+void tbutton::disconnect_signal_mouse_left_click(
+		const event::tsignal_function& signal)
+{
+	disconnect_signal<event::LEFT_BUTTON_CLICK>(signal);
+}
+
 #ifdef GUI2_OLD_EVENT_HANDLING
 void tbutton::mouse_enter(tevent_handler&)
 {

src/gui/widgets/button.hpp

@@ -27,9 +27,31 @@ class tbutton : public tcontrol
 public:
 	tbutton();
 
+	/** @deprecated use (dis)connect_signal_mouse_left_click instead. */
 	void set_callback_mouse_left_click(void (*callback) (twidget*))
 		{ callback_mouse_left_click_ = callback; }
 
+	/**
+	 * Connects a signal handler for a left mouse button click.
+	 *
+	 * @todo Implement everywhere.
+	 *
+	 * The signal should be used for all common cases (which now have
+	 * set_callback_xxx). The those set functions can be removed.
+	 *
+	 * @param signal              The signal to connect.
+	 */
+	void connect_signal_mouse_left_click(const event::tsignal_function& signal);
+
+	/**
+	 * Disconnects a signal handler for a left mouse button click.
+	 *
+	 * @param signal              The signal to disconnect (should be the same
+	 *                            as send to the connect call.
+	 */
+	void disconnect_signal_mouse_left_click(
+			const event::tsignal_function& signal);
+
 	/***** ***** ***** ***** Inherited ***** ***** ***** *****/
 
 #ifdef GUI2_OLD_EVENT_HANDLING