Improve the reference panel

[vkbo]

The reference panel below the document viewer could do with a bit of an update. It is a very old feature that hasn't been given much attention since it was first added. It is a bit confusing, and so are the two buttons "References" and "Sticky" associated with it. See comments in #923.

I propose to:

• Add a more obvious show/hide button for the panel.
• Move the "Sticky" button into the panel itself, and add a descriptive heading on the panel with the sticky button next to it.
• Add some placeholder text in the case where there are no references to show.
• Consider changing how the references are listed, perhaps with a compact/detailed view option.

[tmarplatt]

By default the open reference panel extends to considerable screen real estate, for what it currently holds. Can its height be dynamically set depending on its contents?

[vkbo]

The vertical size of the panel is adjustable. The default size is a proportion of the window height, but it should remember its previous setting after that.

[vkbo]

If anyone's interested, I'm working on the new reference panel functionality for 2.2 now. Here's a video of the code so far. Still has a few issues, but mostly works as intended. This also demonstrates the auto-complete feature for references.

Screencast.from.2023-11-15.18-44-28.mp4