GUI2/Tree View: use NOTIFY_MODIFIED events instead of manual callbacks

Unlike other widgets where `this` is the event target, the tree nodes fire the event
with the tree widget itself as the target.

I had added a fire-event call in f97dc8ae12, but with
the target as the node. I don't think there was any way that would have worked...
changed.

src/gui/dialogs/campaign_selection.cpp

@@ -201,7 +201,8 @@ void campaign_selection::pre_show(window& window)
 	/***** Setup campaign tree. *****/
 	tree_view& tree = find_widget<tree_view>(&window, "campaign_tree", false);
 
-	tree.set_selection_change_callback(std::bind(&campaign_selection::campaign_selected, this, std::ref(window)));
+	connect_signal_notify_modified(tree,
+		std::bind(&campaign_selection::campaign_selected, this, std::ref(window)));
 
 	toggle_button& sort_name = find_widget<toggle_button>(&window, "sort_name", false);
 	toggle_button& sort_time = find_widget<toggle_button>(&window, "sort_time", false);

src/gui/dialogs/gamestate_inspector.cpp

@@ -396,7 +396,8 @@ public:
 		auto left_button = find_widget<button>(&window, "page_left", false, true);
 		auto right_button = find_widget<button>(&window, "page_right", false, true);
 
-		stuff_list->set_selection_change_callback(std::bind(&gamestate_inspector::controller::handle_stuff_list_item_clicked, this, _1));
+		connect_signal_notify_modified(*stuff_list,
+			std::bind(&gamestate_inspector::controller::handle_stuff_list_item_clicked, this, _1));
 
 		connect_signal_mouse_left_click(
 				*copy_button,

src/gui/dialogs/help_browser.cpp

@@ -56,7 +56,8 @@ void help_browser::pre_show(window& window)
 	connect_signal_mouse_left_click(next_button,
 		std::bind(&help_browser::on_history_navigate, this, std::ref(window), false));
 
-	topic_tree.set_selection_change_callback(std::bind(&help_browser::on_topic_select, this, std::ref(window)));
+	connect_signal_notify_modified(topic_tree,
+		std::bind(&help_browser::on_topic_select, this, std::ref(window)));
 
 	window.keyboard_capture(&topic_tree);
 

src/gui/widgets/tree_view.cpp

@@ -40,7 +40,6 @@ tree_view::tree_view(const implementation::builder_tree_view& builder)
 	, need_layout_(false)
 	, root_node_(new tree_view_node("root", nullptr, *this, std::map<std::string, string_map>()))
 	, selected_item_(nullptr)
-	, selection_change_callback_()
 {
 	connect_signal<event::LEFT_BUTTON_DOWN>(
 		std::bind(&tree_view::signal_handler_left_button_down, this, _2), event::dispatcher::back_pre_child);

src/gui/widgets/tree_view.hpp

@@ -97,11 +97,6 @@ public:
 		return selected_item_;
 	}
 
-	void set_selection_change_callback(std::function<void(widget&)> callback)
-	{
-		selection_change_callback_ = callback;
-	}
-
 	const std::vector<node_definition>& get_node_definitions() const
 	{
 		return node_definitions_;
@@ -138,8 +133,6 @@ private:
 
 	tree_view_node* selected_item_;
 
-	std::function<void(widget&)> selection_change_callback_;
-
 	/**
 	 * Resizes the content.
 	 *

src/gui/widgets/tree_view_node.cpp

@@ -577,7 +577,7 @@ void tree_view_node::signal_handler_left_button_click(const event::ui_event even
 	unfolded_ = unfolded_new;
 	is_folded() ? fold_internal() : unfold_internal();
 
-	fire(event::NOTIFY_MODIFIED, *this, nullptr);
+	fire(event::NOTIFY_MODIFIED, get_tree_view(), nullptr);
 }
 
 void tree_view_node::signal_handler_label_left_button_click(const event::ui_event event, bool& handled, bool& halt)
@@ -601,9 +601,7 @@ void tree_view_node::signal_handler_label_left_button_click(const event::ui_even
 
 	get_tree_view().selected_item_ = this;
 
-	if(get_tree_view().selection_change_callback_) {
-		get_tree_view().selection_change_callback_(get_tree_view());
-	}
+	fire(event::NOTIFY_MODIFIED, get_tree_view(), nullptr);
 }
 
 void tree_view_node::init_grid(grid* g, const std::map<std::string /* widget id */, string_map>& data)
@@ -792,9 +790,7 @@ void tree_view_node::select_node(bool expand_parents)
 
 	get_tree_view().selected_item_ = this;
 
-	if(get_tree_view().selection_change_callback_) {
-		get_tree_view().selection_change_callback_(get_tree_view());
-	}
+	fire(event::NOTIFY_MODIFIED, get_tree_view(), nullptr);
 
 	label_->set_value_bool(true);
 }

src/scripting/lua_gui2.cpp

@@ -746,7 +746,7 @@ int intf_set_dialog_callback(lua_State* L)
 		static dialog_callback_wrapper wrapper;
 		connect_signal_notify_modified(*l, std::bind(&dialog_callback_wrapper::forward, wrapper, w));
 	} else if(gui2::tree_view* tv = dynamic_cast<gui2::tree_view*>(w)) {
-		tv->set_selection_change_callback(&dialog_callback);
+		connect_signal_notify_modified(*tv, std::bind(dialog_callback, _1));
 	} else {
 		return luaL_argerror(L, lua_gettop(L), "unsupported widget");
 	}