Clean-up and refactoring

Changes
* Create utility class `Resources` for common operation on rdf resources
* Make use of `Resources` instead of `VisualItem`.
* Remove package `gndata.app.ui.metadata.tree`.
* Create comparators as replacement for `VisualItem.renderForSorting`.

Todo:
* There are still places where utilities from Resources can replace / shorten some code.
* Listen for model changes (relevant for import).