Add positionable onto the field

[sagitta-ignis]

As a player, I'd like to add a new position onto the field, In order to define the (azimuth, distance) from/to another position later.

• Create data :
• [x] interface Positionable
• and each model for :
• [x] Target,
• [x] Spotter,
• [x] Artillery.
• Create dialog for encoding :
• [ ] Target,
• [x] Spotter,
• [ ] Artillery.