altosui: Run all igniter status requests from non-GUI thread

Anything run from the UI thread blocks the UI entirely; the Fire
Igniters startup code to collect the number of pyro channels when
building the UI was doing that from the UI thread. Switch that around
so that the UI doesn't get built until that reply comes back, allowing
the user to see the 'connecting' dialog, and cancel it if required.

Signed-off-by: Keith Packard <keithp@keithp.com>