Enable UI scaling in the scenario and implement Linux menus

Patch from @x-qq
Addresses #195

Note: UI scaling for the scenario editor still needs some work,
but it's basically functional.

src/scenedit/scen.menus.mac.mm

@@ -164,6 +164,13 @@ void update_edit_menu() {
 	}
 }
 
+bool menuBarProcessEvent(const sf::Event&) {
+	return false;
+}
+
+void drawMenuBar() {
+}
+
 @implementation MenuHandler
 -(void) menuChoice:(id) sender {
 	handle_menu_choice(eMenu([[sender representedObject] intValue]));