Update the GUI enabled/paused state if it changes externally

Authored by marten on Feb 8 2020, 2:09 PM.

Description

Update the GUI enabled/paused state if it changes externally

This means via the cupsenable/cupsdisable commands, CUPS detecting an
error etc. Eliminate the explicit tracking of the printer state using
isPaused and simply do it declaratively instead.

Differential Revision: https://phabricator.kde.org/D25186