Remove scrollbar from being a control

[dlamkins]

Having the scrollbar be its own control has caused problems. Most notably with us binding its size and position to the side of the container.

Plan is to split this implementation out as part of the container. Perhaps in a similar spirit to how the LoadingSpiralUtil works, but for the most part containers

[entrhopi]

#392 should be considered when refactoring scrollbar

[dlamkins]

Good catch. 👍

[entrhopi]

I noticed (aside from other issues I have with the scrollbar) that, if we set ShowBorder = false the scollbar will be invisible too.
Is that expected behavior?

[entrhopi]

This should be considered too:
https://github.com/blish-hud/Blish-HUD/blob/dev/Blish%20HUD/Controls/Panel.cs#L266-L267