STSM: basic classes for document search

- inherited from recommendation classes since it's mainly the same
- added a search documents button to the workspace tree
- added a container (later for keywords and inputtext) to the north of the recommendations
not working! search for && recommend documents
STSM: tried to fix - not yet working as intended
STSM: recs are displayed and removed correctly

- search view is not removed correctly
- search view misses the search panel somehow
STSM: working recommendations and search view
STSM: views are now customizable
STSM: heavy refactoring and renaming
STSM: refactoring
STSM: search is working :)

todo:
- bug fixing
- use search models
- download documents
- save ratings
STSM: various minor bug fixes
STSM: reordered retrieval nodes in workspace tree