Update slide up animation implementation

[mixxorz]

Issue ticket #XXXX

If applied, this PR will... Replace animations with libeasing for slide up animations.

Please provide detail on the technical changes, if relevant Previously, the slide up animation was accomplished by animating the slider (using AnimationGroup). With this change, the animation is accomplished by using LibEasing on SetVerticalScroll. This was done because doing the previous approach didn't animate text icons (WoW UI API limitation). Animating the scroll position with LibEasing gets around this issue.

I haven't noticed any noticeable difference between using LibEasing vs normal animations.

Also fix an issue with scroll logic being broken after the frame is resized.

Dev checklist

• [x] Feature works with other addons enabled
• [x] Feature works with all other addons disabled
• [x] README updated (if necessary)
• [ ] CHANGELOG updated