altosui: Allow TM config connection to be canceled.

This leaves the config UI connection attempt running and pops up a
dialog box when it takes 'too long' in the remote case so that users
with Tm or Tn devices can bring up the UI, and then boot the Tm/Tn
without needing to time things carefully.

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