TODO: Discuss the location of immutable-state in pet-file
Adds a custom GroupBox with gray-out capabilities (So this is MR gray out calibration 6/6)
This MR adds a new PGroupBox which is like a normal QGroupBox, except that it allows the user to disable all widgets inside it, such that they are not changed by accident.
To draw the GroupBox like the "native" one, it was neccessary to introduce a helper class PGroupBoxFrame, which overrides its paint event to draw itself like the real frame of a GroupBox. A normal frame cannot be adapted to always have the same appearance.
