Allow PianoRoll to access ClipView of current clip

Currently, the `PianoRoll` only has access to the `Clip` object of the midi clip currently being edited, not the `ClipView`. This is fine for the most part, but it means that certain things about the clip which are stored in the `ClipView` cannot be edited.

In particular, once #7477 gets merged, it would be nice to have a way to toggle auto-resizing of the current clip in the piano roll, since it could get a bit confusing for some users who resize a clip, then try to edit it and see that it doesn't automatically resize.

In order to do this, we could either:
1. Store a pointer to the `ClipView` along with the pointer to the `Clip` in the PianoRoll
2. Only store a pointer to the `ClipView`, and use `ClipView::getClip` to access the clip.
3. Add a method to `Clip` to get which `ClipView` corresponds to it. You can already to the opposite, get the `Clip` via `ClipView::getClip()`, but not the other way around.
4. Store pointers to each clip/clipview in a big list somewhere?

All of these options have drawbacks, either by breaking the desired core/gui separation, or introducing potential sync issues. I would love to hear your opinions on it.

Start by reviewing how PianoRoll currently accesses the current Clip and how ClipView::getClip provides the reverse lookup. Consider the four ownership and lookup options in the issue, including core/GUI separation and synchronization, then define a decision that enables the current clip's auto-resizing control after #7477 without introducing stale pointers.

Written by the indexing model from the issue text.