Use custom splitter component

[mstange]

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I've been playing with a different splitter implementation which lets us use max-height on the timeline, so that we don't have to compute the timeline height based on the visible tracks.

It's working great so far and lets us remove a fair amount of code, and it's another solution to the ResizeObserver warning.

I haven't implemented the panelLayoutGeneration incrementing. The next step would be to find out whether that's still needed or whether we can remove panelLayoutGeneration as well (or maybe replace it with another ResizeObserver).

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