Improve loop boundary UX

[NyuBlara]

Dragging flags can be cumbersome.

Ideas :

• Larger boundary markers (as evoked with Jean in November : big semicircles encompassing the smaller index cursor)
• Keyboard bindings to move each boundary (Ctrl-Left + Left/Right for start, Ctrl-Right + Left/Right for end ?)
• "=" buttons between "Start" and "Current" / "Current" and "End" : brings the two fields to the same value
• Keyboard bindings for above function (similarly, Ctrl-Left + = key to sync Start and Current, Ctrl-Right + = key to sync Current and End ?)
• Minor tweak : When pushing the start value forward, the index value naturally follows it. It should also follow it back if it hasn't been moved away from it, as it currently doesn't.

[NyuBlara]

(crude misaligned illustration for bullet point 3)

We could consider having the "=" signs only when the values are already equal. Otherwise, have a bidirectional arrow, and clicking on an arrow head will copy the value in that direction (example : click right arrow head between Start and Current copies Start value to Current, changing the bidirectional arrow into an = sign)