Add a button to grab easily workers in the in game menu

fixes #84

Concerning the worker pickup, the logic is :
1 - Take idle worker
2 - Take fighting/fleeing
3 - Take claimers
4 - Take diggers
5 - Take gold digger/depositers or all the rest
For each, we pickup the highest leveled available

Concerning the fighter pickup, the logic is :
1 - Take fleeing fighters
2 - Take idle fighters
3 - Take busy (working/eating) fighters
4 - Then take fighting fighters or all the rest
For each, we pickup the highest leveled available

As a bonus, I've added support to add creatures in the editor. To use it :
- Press C to change creature type (class will be displayed in the top of the screen)
- Press Y to change the creature's Seat
- Press fighter button to add the creature. It will be in the keeper hand
- Drop the creature where you want