Disable "Revert" menu item when the patch is not saved.

src/app/MenuBar.cpp

@@ -185,6 +185,7 @@ struct FileButton : MenuButton {
 		RevertItem* revertItem = new RevertItem;
 		revertItem->text = "Revert";
 		revertItem->rightText = RACK_MOD_CTRL_NAME "+" RACK_MOD_SHIFT_NAME "+O";
+		revertItem->disabled = (APP->patch->path == "");
 		menu->addChild(revertItem);
 
 		QuitItem* quitItem = new QuitItem;