Vehicle magazine

Work on https://github.com/KoffeinFlummi/AGM/issues/335

Current Issues:
- The vehicle's weapon's magazine is not checked for max ammo count but the unit's magazine is. (impossible to load more than 1 round and the vehicle has to have used all rounds).
- If the vehicle is set to 0 ammunition in the editor it is impossible to reload the vehicle due to no magazine being availabe. Once the fix for the above is implemented it should be possible to get the loaded magazine for the vehicle, if that is unavailable then doing some magic (I guess) could make it possible for a magazine to be placed in the vehicle and set to the correct ammo count for the loaded magazine.