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).