https://bugzilla.gnome.org/show_bug.cgi?id=770242
* with high key binding configurability which requires no application
* or toolkit side changes.
*
+ * In order for bindings to work in a custom widget implementation, the
+ * widget’s #GtkWidget:can-focus and #GtkWidget:has-focus properties
+ * must both be true. For example, by calling gtk_widget_set_can_focus()
+ * in the widget’s initialisation function; and by calling
+ * gtk_widget_grab_focus() when the widget is clicked.
+ *
* # Installing a key binding
*
* A CSS file binding consists of a “binding-set” definition and a match