Refactor model: LayerItem

t.knopffrequested to merge
refactor_model into develop

In this MR the interface of LayerItem has been overhauled to disencourage the use of the string based interface.

While refactoring a new class ItemWithMaterial was introduced to serve as an abstract base class for LayerItem and ParticleItem because both have a "material" property and some parts of the code could be made simplier by using this class.

Relates #52 (closed)

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