Set icon class to sidebar-icon, and label class to
sidebar-label. This makes it more consistent with
places sidebar in Nautilus and GtkFileChooser.
https://bugzilla.gnome.org/show_bug.cgi?id=786612
As the sidelist gets more complex, managing it in CcWindow
would make it very confusing.
This patch introduces the CcPanelList, a widget that manages
the sidelist.
https://bugzilla.gnome.org/show_bug.cgi?id=767301