[Lens] collapsable layer panels

[ghudgins]

Describe the feature: Ability to visually collapse layer panels in the Lens editor to browse more at once

Design questions / how far to go / brainstorm:

• How many are open by default? 2? How does that change as they are added?
• How do collapsed layers interact with drag & drop? Do they expand on hover?
• Should we also consider drag and drop layers in order or is that something separate?

Describe a specific use case for the feature:
When I have a large number of chart layers, reference lines, and annotations (a growing case now that annotations is in tech preview at 8.2)
I need a way to collapse panels
So I can navigate the tall layer list more easily

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[stratoula]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.