QoL improvements for parts with a large number of subtypes

[Scialytic]

If there's anything you don't like or want me to change, I'm happy to fix it.

I think this is mostly relevant for Conformal Decals. https://github.com/blowfishpro/B9PartSwitch/issues/199 With a big atlas texture, 50+ (or even 100+) subtypes is not outside the realm of possibility.

The popup selector is unchanged if it fits nicely on screen:

But if it would be too tall, a scrolling version is used instead:

The in-PAW selector now scrolls automatically to keep the current subtype button visible. The selected button will be centered unless it's too near the start/end of the list. I also boost the scroll sensitivity according to the number of buttons, for people who prefer mouse-wheeling to dragging their way through a long list.

[Grimmas]

We were discussing on Discord today and this came up. This is definitely a feature I want to see in B9, as I have a few patches out that convert other mods to B9 and they have very many tank setups to the point that the window overflows and it's impossible to select some of the tanks.