Refactor the game event loop and add menus to the Linux build

Patch from @x-qq
Addresses #195

src/game/boe.menus.mac.mm

@@ -278,6 +278,13 @@ void menu_activate() {
 	[[file_menu itemWithTitle: @"Save As…"] setEnabled: YES];
 }
 
+bool menuBarProcessEvent(const sf::Event&) {
+	return false;
+}
+
+void drawMenuBar() {
+}
+
 @implementation MenuHandler
 -(void) monstMenu:(id) sender {
 	cMonster* monst = [[sender representedObject] monst];