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
