GUI: MaterialItem: derive descriptors from properties

DoubleDescriptors are mostly derived from DoubleProperty in the code.

In MaterialItem this was done differently, so this MR brings it in line with the common approach.

Steps of Descriptor refactoring:

1. reduce usage of non-default functional pointers set() and get()
2. move Descriptor functionality to Property
3. use Property instead of Descriptor
4. remove Descriptor from code
5. refactor serialization of Property
6. replace Property macro definitions like DOUBLE_PROPERTY(intensity, Intensity) by explicit member with setter and getter

This MR does (1) for DoubleDescriptors and VectorDescriptors