Refs #8776. Split signal for any change and auto-update.

We need to show/hide custom data file widget no matter whether the
value was changed programmatically or by user, but auto-update only
if the value was changed by user. So I've split the functionality into
two parts.
Refs #8776. Improve error handling a bit.
Merge remote-tracking branch 'origin/bugfix/8776_muon_dead_times_field_not_hidden'