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.
This is a container widget that takes over all the duties of tree
expanding and collapsing.
It has to be a container so it can capture keybindings while focus is
inside the listitem.
So far, this widget does not allow interacting with it, but it shows the
expander arrow in its correct state.
Also, testlistview uses this widget now instead of implementing
expanding itself.
Due to the many different ways to set factories, it makes sense to
expose them as custom objects.
This makes the actual APIs for the list widgets simpler, because they
can just have a regular "factory" property.
As a convenience function, gtk_list_view_new_with_factory() was added
to make this whole approach easy to use from C.
Previously, we were recreating all widgets every time the list item was
rebound, which caused a lot of extra work every time we scrolled.
Now we keep the widgets around and only set their properties again when
the item changes.
We now don't let the functions create widgets for the item from the
listmodel, instead we hand out a GtkListItem for them to add a widget
to.
GtkListItems are created in advance and can only be filled in by the
binding code by gtk_container_add()ing a widget.
However, they are GObjects, so they can provide properties that the
binding code can make use of - either via notify signals or GBinding.
The thing we're actually doing is create and maintain a widget for every
row. That's it.
Also add a testcase using this. The testcase quickly allocates too many
rows though and then becomes unresponsive though. You have been warned.
