shared-folders: Decouple from SpiceDisplay

The current implementation of Shared Folders was strictly tied to the Spice Display, because spice-webdav is the bridge that makes this feature possible with libvirt machines. But now we could also have shared folders in RDP Remote Machines. Therefore decoupling the shared folders implementation from the spice-display class was a natural choice.

The way it works now is that there's a central singleton object named SharedFoldersManager that interfaces everything that is necessary for the feature to work in the host side (saving the gsettings, creating the symlinks, etc...).

The basic API for the SharedFoldersManager is to query for shared folders with SharedFoldersManager.get_folders (machine_uuid), and add/remove items, since SharedFoldersManager implements the Boxes.Collection interface.

These changes also make the use of cleaner methods of implementing the functionality, such as decoupling the logic and view with Gtk widget templates, and the use of GLib.ListStore models to back the Gtk.ListBox views (giving us free ordering, sorting, and row updates).