First try to adapt project widgets to project API v2
Replace old Project, ProjectManager, Model, ... by API v2 classes
All unit tests pass but this commit break application (yes coverage is too low!)
Remove old project manager widget (never used)
Delete no more used treeview
First try to fix projectwidget actions
Save strings instead of paths in ProjectManager settings
Fix dataflow loading.
Need to clearly define when model content is loaded
Improve content management for model classes:
  - No more need to explicitly load model to use it
  - Define very basic cache system
  - Content in not load at construct but only if required.