Expand timeslices in schedule editor on hover

[rixx]

This PR is very WIP and does not fully work yet. Status: Timeslices can expand on hover, but they flicker and aren't clickable.

Idea:

• new state, tentativeExpandedSlices instead of hoverSlices, which can be made permanent with a click (adding those timeslices to the expanded slices)
• re flickering: rash says to watch hoverSlice, but that's currently only updated when dragging, bc expensive. we could stuff that into updateHoverSlice (rename that function) and bail directly afterwards …

[rixx]

Abandoning: this is super buggy unless you do some serious observing (@mouseleave doesn't really cut it in many cases), and even if it works perfectly, the constant change in schedule size is just janky and unpleasant. I put some other ideas into the tracking issue.