Version 0.99 preliminary, needs firmware 0.78m-dc42

1. Now displays active and standby head temperatures and allows both of
them to be set.
2. Keep the 3 sliders up to date with the values being used on the
printer, e.g. after reconnecting or a printer reset.
3. When connecting, if a file is being printed then fetch its layer and
filament data and switch to the Print Status tab.
Version 0.99 final. Needs firmware 0.78m-dc42.

When reconnecting, if a file is being printed then display the details
on the print Status tab.