Added ability to edit a media source from the files tree

### What does it do ?

Allow users to edit a media source directly from the tree (`?a=source/update`)
Restores CSS classes in the tree giving hints on user permissions

### Why is it needed ?

Provide some consistency in the trees : most of listed data in trees could be editable directly from the tree (contexts, resources...) as well as save some clicks for lazy people :smile: