We were not storing the previous value, causing
the first two suggestions to work, but not later
ones.

Fixes the test added in the previous commit.

Make the 'repeat edit' test make more than to
suggestions in a single edit phase. It turns out
that this does not work, whereas just doing
two in a row does.

Add a max size to the buttons, to force the
space to open up.

This is meant to be a flexible space.

Since GtkWidget implements GtkConstraintTarget, we can omit the explicit
cast, and validate the type at run time.

This is in preparation for allowing non-widgets
to act as constraint targets.

This is an marker interface that we will
use to accept other things that widgets
in constraints.

Changing the set of constraints should cause a relayout.