sharing: Change explanatory text to match mockups

https://bugzilla.gnome.org/show_bug.cgi?id=731580
sharing: Move Media Sharing master toggle to header bar

As per mockups

https://bugzilla.gnome.org/show_bug.cgi?id=731580
sharing: Rename header label in Media Sharing

As per mockups

https://bugzilla.gnome.org/show_bug.cgi?id=731580
sharing: Use GtkListBox instead of treeview for media folders

As per mockups

https://bugzilla.gnome.org/show_bug.cgi?id=731580