Tweak slide up animations

[mixxorz]

Issue ticket #67

If applied, this PR will...
Attempt to fix the jittery slide up animations some players are experiencing

Please provide detail on the technical changes, if relevant
The previous implementation works like this:

1. Scroll frame so that new messages are at the bottom
2. Shift slider frame down instantly (to negate the scroll frame movement)
3. Slide up transition

Steps 1 and 2 don't always happen in the same frame, resulting in some jittering
for players with high FPS.

The new implementation works like this:

1. Slide up animation
2. After the animation finishes, scroll frame so that new messages are at the bottom

Based on my testing, this produced no jittering.

Dev checklist

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