xpra top list view improvements

[Antoine Martin]

Following up #2363.

• use threading so we don't block the main thread querying the list of sessions
• show the return code string when launching a subcommand (especially if it fails)
• give more of an overview: show how loaded the sessions are, sort them by CPU / fps?

[Antoine Martin]

See also #2601

[Antoine Martin]

Updates:

• r27027 better handling of signals, show subprocess exit info
• r27028 show server cpu percentage
• r27029 refactoring
• r27030 win32 and macos packaging for psutil

TODO:

• ensure we never try to use a remote server pid... (hard)
• expose CPU usage via xpra info instead?

[Antoine Martin]

r27034 ensures that the server is local before querying CPU usage using the pid.

This will do.