add thread view

[tromey]

It would be nice to have a thread view. It would show all the inferiors and their threads, along with their running states. Perhaps drilling down further into a thread could show its stack; though it might be better to have a separate stack window.

Note that there's no good way to note new threads: https://sourceware.org/bugzilla/show_bug.cgi?id=15622

It might be sufficient as a stopgap to only update the display on a stop, and not worry about async and scheduler locking and the other modes.