display-page-tree: copy layout from GtkPlacesSidebar, remove expanders

Expanders in the top level groups add a fair amount of clutter and
aren't really all that useful.  The tree is generally ordered so that
even if you have lots of playlists, network shares and devices, you
don't need to scroll through them constantly.

data/org.gnome.rhythmbox.gschema.xml

@@ -137,13 +137,6 @@
       <description>The list of columns that will be shown. If a given source doesn't support a particular column, it won't be displayed.</description>
     </key>
   </schema>
-  <schema id="org.gnome.rhythmbox.display-page-tree" path="/org/gnome/rhythmbox/display-page-tree/">
-    <key name="collapsed-groups" type="as">
-      <default>[]</default>
-      <summary>List of collapsed page groups</summary>
-      <description>The list of page groups that will initially be collapsed rather than expanded.</description>
-    </key>
-  </schema>
   <schema id="org.gnome.rhythmbox.library" path="/org/gnome/rhythmbox/library/">
     <key name="layout-path" type="s">
       <default>'%aa/%at'</default>