Add the names of the main widgets as keywords to
our demos, but also things like "game". This helps
finding relevant demos in our growing list. You
can now for example type "label", and find the
"error states" and "links" demos showing GtkLabel
features.
Change the apis in GtkListView, GtkColumnView and
GtkGridView to be explicitly about GtkSelectionModel,
to make it obvious that the widgets handle selection.
Update all users.
