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.
The widget mostly works out of the box, but some tweaking may be
necessary (in particular in the theme) and the gtk-demo changes might
require removing before this is production-ready.
This reverts commit 6a164ab306dad9096bde736c907494c71086d3c4.
The function was awkward and we now have only one caller again, so we
can fold it back into it.
Nothing really changes, because both ListView and GridView still keep
self->item_manager around, but it's set up to point at the base's item
manager.
This way we can slowly move things to GtkListBase that need the item
manager (like trackers).
