Many improvements
 - Support both collections of collections and collections of photosets
 - More robust authentification phase and sql storage
 - UI based on StackView
 - Retrieve list of photosets with their icons
 - Include number of items in collections
 - Add a text widget to display the current path, clicking on it goes back in stackview


Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>