listitem: Make this a GObject

listitem: Make this a GObject

This splits GtkListItem into 2 parts:

1. GtkListItem
   This is purely a GObject with public API for developers who want to
   populate lists. There is no chance to cause conflict with GtkWidget
   properties that the list implementation assumed control over and
   defines a clear boundary.
2. GtkListItemWidget
   The widget part of the listitem. This is not only fully in control of
   the list machinery, the machinery can also use different widget
   implementations for different list widgets like I inted to for
   GtkColumnView.