[SIGMON-356] `gui.pc` refactoring

Context

• task: https://its.cern.ch/jira/browse/SIGMON-356

What was done:

• the code was refactored (to some extent)
• tests got simplified

QA

• sonar is not happy, but we can discuss: http://sonar/project/issues?branch=SIGMON-356_gui_pc_refactoring&id=lhc-sm-api&resolved=false