Setting a width request is not quite enough, since
gtk_widget_set_size_request() only queues a resize
when the widget is visible. Explicitly force one
here. Without this, the popup sometimes shows up
too small.
This is a list model holding strings, initialized
from a char **. String lists are buildable as well,
and that replaces the buildable support in GktDropDowns.
Since it's a type with sub-classes, we need to use GTypeInstance (at the
very least), otherwise we won't be able to address each sub-class as
such.
This is similar to how GskRenderNode and GdkEvent are handled, with the
added difficulty that GtkExpression is meant to be used in properties,
in order to be deserialised by GtkBuilder. This requires adding a
GParamSpec sub-class that we can match on from within GtkBuilder,
alongside some convenience API for storing a GtkExpression inside a
GValue.
