GUI: Move setPolarization from BeamItem to InstrumentItem

I intend to move polarization-related members and functions from classes Beam and IDetector to class Instrument. Instead of Beam::setPolarization, we will have Instrument::setPolarization. Motivation comes from specular reflectometry where we still have polarization filters, though we don't have a fixed incoming beam and a fixed 1- or 2-dimensional detector.

The intended move is currently blocked by BeamItem::createBeam() in BeamItems.cpp. Similarly, we need to take DetectorItem::createDetector() out of DetectorItems.cpp.

This is not mere refactoring, but should be accompanied by a change in GUI layout: There should be a PolFilters view that handles incident and final polarization filters (polarizer and analyzer). For the time being, our instrument view should consist of beam, detector and polfilters views.