Downloading file messages will now show a transfer control above the
composer, similar to uploads. Measuring the progress or pausing the
operation is not possible yet.
Compression with Pillow can take long, especially with large
clipboard PNG images.
Doing this in a separate process prevents the async event loop from
getting blocked, and allows multiple compression operations to run in
parallel.
Use native ListModel which require a lot of changes, but should be
much faster than the old way which exponentially slowed down to a crawl.
Also fix some popup bugs (leave/forget).
Not working yet: side pane keyboard controls, proper highlight,
room & member filtering, local echo replacement
