tool-crop: Restrict use of gtk_combo_box_get_active

There is no way to get the activated row of a GtkListBox without the
GtkListBox::row-activated handler. Restricting the use of
gtk_combo_box_get_active to the GtkComboBox::changed handler paves the
way for migrating from GtkComboBox to GtkListBox.

https://bugzilla.gnome.org/show_bug.cgi?id=758227