Add new data models.
Add module relation to application model.
Add missing hashCode() implementations.
Use isLeaf() for TreeNode hashCode() calculation.
Add missing equals() implementations.
Add missing toString() implementations.
Add comments to the new model classes.
Initialize collections to avoid NullPointerExcptns
Remove undiscussed application properties.
Add additional Layer constructor.

This makes sense as dataSource and defaultTheme are mandatory.
Add attribution property to LayerDataSource
Adapt BaseLayerTheme properties.

Added attribution property and removed undiscussed hue property.
Add LayerTree-constructor with rootNode argument.
Merge pull request #42 from buehner/data-modeling

Introduce new data models