This will allow us to use it in a listmodel

This sets the groundwork for allowing windows to update
their titles dynamically.  At this point we are still
refreshing all of the window (and creating new objects)
on any window change.

Using a listmodel internally will benefit the primary
consumer, screencastwidget.  This way, shellintrospect
will be in charge of all of the window add/remove/changes.

Rather than trying to recreate what shellintrospect is doing,
simply attach the listmodel to the listbox. Use a filter list
model to filter out some of the windows as we were previously
manually doing.

Besides the code benefits, this fixes a user visible issue
where the list completely recreates on any window changes.
This is particularly problematic when a window is selected
(and becomes unselected) on the window change.

That means we'll avoid first emptying the list, then filling it.

This avoids deselecting selected windows.

This makes unselecting a window work again.

It gets too much in the way, and makes it hard to impossible to
implement click-to-toggle as well as not resetting the selection when
using keyboard navigation.

Instead rely on the GtkListBox cursor to navigate, and track selection
state ourself. A tiny CSS class is added to deal with making it look the
same as before.

Closes: https://gitlab.gnome.org/GNOME/xdg-desktop-portal-gnome/-/issues/93