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.win.cpp

@@ -253,3 +253,10 @@ void set_up_apple_events(int argc, char* argv[]) {
 		}
 	}
 }
+
+bool menuBarProcessEvent(const sf::Event&) {
+	return false;
+}
+
+void drawMenuBar() {
+}