Moved all info types into their own directory.
Removed obsolete context.
Added DataProvider interface in backend.
Editor needs to keep the Sequence it's modifying alive.
Modified info types accordingly to the following truth: the Project hold all the data which are then instantiated in the different objects on need.
Minor TaskExecutor tweak.
Workspace now use DataProvider to open projects using a URI.
Also removed the Sequence find-or-create code as it is the Project which should manage this.
Added find_or_load sequence in Project implementation.
WIP - adding dummy DataProvider implementation for backend tests.