Reload directory tree and keep state

[willmcgugan]

We need a way of reloading the directory tree on demand, so that it reflects any changes to the filesystem.

• The reload should be atomic. i.e. it shouldn't permit modifications to the tree from another task. Otherwise it could get out-of-sync. Suggest we need a asyncio.Lock here.
• The highlighted node should stay highlighted, assuming the node still exists. If the node has been removed, then the highlight should go up a directory.
• Open directories should be reloaded recursively.
• The loading directory information (including subdirectories) should happen in a thread, so as not block the main loop. Note that thins should probably be done in a single thread.

Suggested implementation:

• In a thread (or threaded worker), recursively load the directories which have been expanded (assuming they exist).
• Note the highlighted node, and the open nodes.
• Update the DirectoryTree nodes to reflect the new data.
• Restore the highlighted node.

Note than since we want this to be atomic, this should be done in a coroutine, or at least return an awaitable.

Supersedes #3574

[github-actions]

Don't forget to star the repository!

Follow @textualizeio for Textual updates.