Overhaul UI: instrument view (cont.)

This MR contains some more reimplementations of user interface elements in a more specific manner than the existing ones based on SessionItemWidget:

• detector editor, consisting of rectangle detector editor and spherical detector editor
• off specular beam editor and GISAS beam editor
• instrument editor with GISAS instrument editor and off specular instrument editor
• footprint correction editor

requested review from @m.puchner

assigned to @m.puchner

Approving it without diving into the details...

approved this merge request

assigned to @t.knopff and unassigned @m.puchner

mentioned in commit 7e9293fd

merged