Weapon holding

[Fueredoriku]

• A general 2 body inverse kinematics implementation
• Inverse kinematics for weapon holding
• Fixes Fix aiming edgecase #158

[toberge]

Much better than floating guns, but the elbow joints don't seem to move in a sensible way, the arm flips when you aim downwards, and the gun clips through your shoulder:

The reload animation makes it look a little odd as well:

And solar + crosshat... what is he holding?

Same problem with dancer + shotgun:

[Fueredoriku]

I tweaked the hand bone to rotate better with the gunbodies. 😁👍
The rest of the visual quirks either require more computations than they are worth / some would require manually implementing constraints.

Most of this can be mitigated in the current system should future problems arise by further tuning provided offset values, existing animations, or even updating some of the gunbodies to have better scaling or dropping two-handed carrying if we decide we don't want arm clipping through body at all.

A fix altogether could be to have different models in first person compared to what others see, which would make for less extreme gun position distances relative to the player. This feature would also inherently solve a lot of the other aforementioned quirks, but we'll have to evaluate if such a feature should be prioritized later 😅

For now I'll merge this in!