Improved slicing and file management

- File management is prepared to support folders (UI still lacking support)
- STL files are now first class citizens
  - can be managed via the file list (and filtered from it)
  - can be sliced on demand
- Slicing now happens via plugins
- Added Cura plugin
  - uses CuraEngine directly instead of full Cura installation
  - Cura profile importer for importing profiles from regular Cura installs via settings dialog
  - TODO: UI for editing profiles
- API for slicing and slicing profile management