Figure counting functions

[Johannes-callidus]

This PR introduces new functions to count figures and rework figure categories to a bitflag enum and them as argument for the counting functions.

[Sephirex95]

I was thinking about this one yesterday - I can merge that, but I would prefer if we had a proper figure counting function for areas, that would allow counting of:

• All figures
• Only Enemy
• Only Citizens

Since that would be useful in more situations, and would allow us to check for figures more precisely. Outside of flotsam and gulls, of the top of my head, explosions and smoke are also figures, and I think so are projectiles?
It would help with counting we do before many other actions connected to building and repairing

[Johannes-callidus]

Which figures exactly do you mean by citizens. Legions as well?

[Sephirex95]

I meant human/animal figures excluding enemy.
So yeah, I'd include legions.

However, if you have time:
Ideally we should have categories in a bitflag enum, so you can pass a combined 'figure_mask', like:
Figures_citizens+ figures_legions+neutral_animals

[Johannes-callidus]

But only with figure "groups" because there are to many figure types to implement that.

[crudelios]

Codewise this looks good.

@Sephirex95 can you check this please since you had some suggestions?