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