Add a recently open documents panel

[vkbo]

Prompted by #1432
Replaces #1292 if implemented

It may be useful, especially for large projects, to have a recently opened documents panel below the project tree. The panel must be independent from the project/novel tree view switching, and must be collapsible.

The panel should:

• List documents with the latest on top.
• Allow the user to sticky documents.
• Allow the user to remove documents from the list.
• Allow the user to only add edited documents to the list rather than opened documents.
• Documents that aren't stickied can be cleared on restart.
• The tool should cache the QPlainTextDocument for at least a good while to preserve the Undo/Redo history.

[tmarplatt]

Some linux DEs are configured to enforce privacy rules on software suites, which include controlling the behaviour of lists of recently opened files. E.g. GNOME does this with gedit, nautilus and such. I think KDE has something similar? Is it perhaps something to consider here?

[vkbo]

That's a sensible policy, but I can't see how it's relevant to recently opened documents within the project.

As for the Open Project dialog with the recent projects list, that may be relevant.