Warn about problems with island maps

[Aviv]

Island maps are available to users even though some components of the game do not fully support them, and performance is harmed on these maps.

Users should be notified of these problems with island maps ahead of time with messages such as: "This is an island map. Computer opponents cannot play very well on island maps, and you may experience increased lag on these maps as well. Thus, island maps are best played on multiplayer or sandbox mode."

[fabio]

See also #1305.

[Deiz]

Fixed in r12326.