Refactor material model
Main aim of this MR is to change "one material list for all samples" to "one material list per sample".
"One material list for all samples" has a few major drawbacks:
- Editing materials of one sample potentially affects other samples. This is critical because the user is very likely not aware of this.
- Undo/Redo is related to one sample. If there is a change in materials, other samples would be affected.
- Importing samples potentially affects existing samples (see #49 (closed))
The original idea of such single material list may have been something like providing a material library. However, this would have to be realized differently - and with all the done refactoring would be quite easy now (see also #155 (closed)).
The refactoring includes cleanup of code, like rename MaterialModel -> MaterialItems, since it is not a model any more and would lead to confusion. Also the wrong dependency (#171) has been rectified.
This fixes #49 (closed)
This fixes #171
