Hosting hint to forward the UDP port

[elexis]

Until #2305 is complete, the hosting dialog should show a hint that the selected UDP port has to be forwarded on the router for the game to be accessible.

The lobby welcome message contains this hint, but that message is only visible until a game was selected (presumably, that's one of the first things to do when joining the lobby).

By adding it to the dialog in gamesetup_mp.xml, it will be shown for both lobbied- and non-lobbied games. (Thus the hint should specifically address internet hosting, since LANs and VPNs don't need that port forwarding on a router).