Forecast/Stats Window: Add "Fuel"

[Slobjok]

We already have "Gold". Same is expected by some players for "Fuel", even though the consumption with units is of course much more volatile than gold. An implementation just like gold is easy. But: for fuel probably mobile units carrying fuel need to be included in the storage capacity as well. And worse: It's not enough to just assume every power generating building (especially generators) is always running. The future power need must be considered to know the future fuel need for each future turn. That probably requires some refactoring to reuse the logic from the base management.

[iindex]

I would consider fuel in mobile units as a separate field somewhere in the dialogue. For forecast just use fuel available in the base, but just for awareness include that there is in total that amount of fuel in mobile units for the player to consider that and maybe transfer fuel back to the base.