On MacOS, `tempfile.mkdtemp()` doesn't necessarily return a canonical
path (it uses `/private/var` instead of `/var`, but they are the same
thing). Since the `filemanager.storage` module canonicalizes paths,
tests were erroneously failing due to prefix match failures.
- 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
