[interface] auto-confirm dialogs

[cuchulain]

I am missing just one important thing. In multiplayer often game is stopped due dialog, so my turn is not initiated and game is silently stopped. This is unpleasant if I do something on my pc and I am not concentrated on game (wesnoth window is not on top of system desktop) and no gong warn me that it is my turn, other players must wait. I suggest

1. to add gong on newly shown dialogs (optional in prefs)
2. autoaccept dialogs choice in preferences (better solution)

one of these solve problem, both will be nice

[Ravana]

While messages would mostly be design choice of scenario authors, it should be possible to make any [message] make sound by editing the tags lua definition.

[Pentarctagon]

I don't know about Mac, but there is already an alert that pops up when it's your turn on Windows/Linux.

[zookeeper]

Sounds like [message] should probably trigger taskbar notifications (although I don't know if/how those work on platforms other than Windows).