Virtual scrolling in the layer tree

With 10_000 layers, the layer tree takes 8s to render. This is obviously somewhat unacceptable.

By only creating DOM nodes for the layer rows that are visible (usually 10-20) the performance could be improved and could become independent of the number of layers. Since all of the layer rows are the same height it would be trivial to add a spacer at the top and bottom to ensure the scroll bars remain the correct height.

Working towards #4572.

Relevant code: https://github.com/GraphiteEditor/Graphite/blob/55c4e6689b3a99e56749280b9694ffac4037c3e7/frontend/src/components/panels/Layers.svelte#L567-L571