Show a warning if the instance path contains a '!'

The checks and warnings happen the time MMC loads (via QLOG_INFO), the time the GUI starts (via a dialog) and when the user changes the instance path via the settings window.

main.cpp

@@ -17,6 +17,7 @@ int main_gui(MultiMC &app)
 	mainWin.show();
 	mainWin.checkMigrateLegacyAssets();
 	mainWin.checkSetDefaultJava();
+	mainWin.checkInstancePathForProblems();
 	return app.exec();
 }