New slicer option: Q slicer

[astellhorn]

Add new slicer option: "Q slicer"

• need sector box and annular box --> convolute them
• get a Q slicer box looking like in the attached image
• optimum: similar as requested in Batch fitting options #2077 it should be made possible to add several boxes as shown in the image below.
  (i) either with d_phi/ dQ completely user defined for EACH BOX
  (ii) or with d_phi/ dQ mirrored at Qx=0 / Qy=0
  (iii) or with a defined rotation in phi from box to box (e.g. Box 1 at phi1, Box 2 at phi1+45deg...)

[butlerpd]

The first part of this has been done and is in 6.0. Only the last part (providing multiple slicer widgets on a single plot) remains and is actively being discussed though not yet worked on.