New ability: submerge

[Jetrel]

I just finish a patch that add the submerge ability

I submit soon this patch to Savannah if you are agree with my implementation.

This patch add a new ability "submerge" very similar to "ambush" ability.
This ability runs invisibility for a unit that :
- has "ability=submerge" in its .cfg
- is in "sea" case (terrain's code 's' and 'c')

It seems to work well for me but it needs to be tested more intensively.

Cool, however, we really need an AI that can deal with invisible units properly. If you could help with making that, which we really need, then some very cool things can happen.

Right now, we *kinda* avoid use of invisibility abilities, because the AI has such a hard time with them.