GUI: rm SelectionDescriptor for detector alignment

Rectangular detector alignment is now handled without creating a SelectionDescriptor.

Steps of SelectionProperty/Descriptor refactoring:

1. make SelectionProperty less generic, make catalogs usage mandatory, not optional
2. reduce usage of ad hoc SelectionDescriptors, e.g. descriptors that are not derived from SelectionProperty
3. move Descriptor functionality to Property
4. use Property instead of Descriptor
5. remove Descriptor from code
6. refactor serialization of Property

This MR is related to points 2.