Separate Media List into a Backone View

Selected Medium -> Active Medium.
Dummy "Chooose One" Medium becomes a real one, send by the server.
Push the selected "Medium" in the App models

It's actually not a medium but a combinaison of the sections, selected section and medium.
Need to split that.
Record active section on the App's models

Change it when selected.
Set a default when medium is selected.
Factorize more code between root and regular section code in tree selection event handler.
Differentiate showing and active Section

Showing is the one feeling up the seeker slider and active is the one edited by the interval slider.
Fix bug where the label of start and end was not updated anymore.
Select root section when right clicking on it (instead of just double selecting it).