emojier: Rework sidebar

Oliver Beardrequested to merge
work/olib-tp/emojier-sidebar into master

This MR moves the sidebar open/close action to the header, simplifies logic and sets the correct height for the sidebar to not scroll (rather than making a guess and adding to it).

Issues with initially animating and the sidebar's width appear to be caused by Kirigami itself, rather than being a bug here.

Before After
image.png image.png

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