FlowListDelegate: fixed and selection improvement. Integration to the PhotoSetPage.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
FlickrAPI: introduce a safe-guard to ensure that we don't call the callback when the Item that initiated the Flickr call has been destroyed.
This lets us use the MouseArea for click and double-click again, as the spurious click event is now handled smoothly.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
ContextPanel: rework a bit the layout and introduce smoothed transitions

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
Navigation: introduce BrowserPage, which is now the base class for all pages. It contains the model, the id and the type of the displayed content.
Also adapt the existing code to take this change into account.
It is now possible to display the properties of the currently shown page when there is no selection.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
Authoring: implement create and move of collections.

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