Implementation of server side FOW

Merry christmas everybody ^^

As a christmas gift, here is the PR implementing FOW.
This PR also contains a lot of small fixes / improvements.

Known bugs:
- When an ennemy claims your last visible tile, you loose vision but the tile is still displayed with your color (I will create an issue for that because this commit was big enough) #395 
- Creature vision is not well handled (that's really visible now that it is used for FOW - I will create an issue) #396 
- The not visible tiles are displayed like the visible ones so it's not obvious which tiles you have vision on or not

This needs to be tested well.
Please note that on the 2v2 skirmish test map, I've added a dead creature and an ennemy kobold to see what happens when you loose vision on a kobold carrying a dead corpse.

@Bertram25 Sorry for the big PR to review

@oyvindln Don't hesitate to have a look if you can

@akien-mga As I said, this PR changes the architecture of most of the game so it can introduce regressions nearly everywhere. Please don't hesitate to test it as much as you can ^^

Enjoy ^^