Filter UI threading

[Ryan McNally]

I've noticed that the swing filter UI briefly gets a bit janky after you choose tags for the first time, when flows are being built. This is obviously a bit lame, we're probably doing the flow construction on the AWT thread or something.
We should make sure that flow construction is on some background thread so that the UI remains responsive. It might be worth putting up a progress bar or something too.

While we're about it we could do the same for the CLI filter dialog.