attempt to fix 'mouseover' state not being selected

when the mouse is inside a child widget of a toggle_panel.

This currently only fixes it for label, image and grid childs. Maybe we
want can_mouse_focus to be false by default?

src/gui/auxiliary/event/distributor.cpp

@@ -175,6 +175,9 @@ void tmouse_motion::signal_handler_sdl_mouse_motion(const event::tevent event,
 		}
 	} else {
 		twidget* mouse_over = owner_.find_at(coordinate, true);
+		while(mouse_over && !mouse_over->can_mouse_focus() && mouse_over->parent()) {
+			mouse_over = mouse_over->parent();
+		}
 		if(mouse_over) {
 			DBG_GUI_E << LOG_HEADER << "Firing: " << event << ".\n";
 			if(owner_.fire(event, *mouse_over, coordinate)) {

src/gui/widgets/grid.hpp

@@ -184,6 +184,7 @@ public:
 		return child(row, col).widget();
 	}
 
+	virtual bool can_mouse_focus() const OVERRIDE { return false; }
 	/***** ***** ***** ***** layout functions ***** ***** ***** *****/
 
 	/** See @ref twidget::layout_initialise. */

src/gui/widgets/image.hpp

@@ -54,6 +54,8 @@ public:
 		return label();
 	}
 
+	virtual bool can_mouse_focus() const OVERRIDE { return false; }
+
 	/***** ***** ***** ***** layout functions ***** ***** ***** *****/
 
 private:

src/gui/widgets/label.hpp

@@ -63,6 +63,7 @@ public:
 
 	void set_link_color(const std::string & color);
 
+	virtual bool can_mouse_focus() const OVERRIDE { return false; }
 private:
 	/**
 	 * Possible states of the widget.

src/gui/widgets/widget.hpp

@@ -341,6 +341,10 @@ private:
 	virtual tpoint calculate_best_size() const = 0;
 
 public:
+	/**
+	 * Whether the mouse move/click event go 'through' this widget.
+	 */
+	virtual bool can_mouse_focus() const { return true; }
 	/**
 	 * Can the widget wrap.
 	 *