Explorer needs a Spinner

[Martitza]

Bazaar Explorer needs a spinner, similar to what several qbzr components already do.

There are times when bzr explorer is extremely unresponsive.

This problem might be reduced or eliminated by the recently proposed effort to factor explorer into multiple threads.

But it might also be good design to anticipate that there might still be circumstances in which bzr explorer gets bogged down.

I know some people think spinners are "idiot lights." We ignore them. But some users really need to be reassured than an application which has not repainted in over two seconds is actually still doing something. So I'm suggesting a spinner, which is different from a progress bar. A spinner just says "Trust me, I'm still doing something, and if you close me you deserve whatever you get."