[gtk/wip/ebassi/constraint-layout] Add GtkConstraintTarget

[Emmanuele Bassi <ebassi src gnome org>]

commit 02f4e1a5aed0c6d15e71214ee3289179745ed7e2
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jun 25 15:04:05 2019 -0400

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

 gtk/gtkconstraint.c | 14 ++++++++++++++
 gtk/gtkconstraint.h | 15 +++++++++++++++
 2 files changed, 29 insertions(+)
---
diff --git a/gtk/gtkconstraint.c b/gtk/gtkconstraint.c
index d0c152b56d..d7bf3e0a08 100644
--- a/gtk/gtkconstraint.c
+++ b/gtk/gtkconstraint.c
@@ -581,3 +581,17 @@ gtk_constraint_detach (GtkConstraint *constraint)
   constraint->constraint_ref = NULL;
   constraint->solver = NULL;
 }
+
+typedef struct _GtkConstraintTargetInterface GtkConstraintTargetInterface;
+
+struct _GtkConstraintTargetInterface
+{
+  GTypeInterface g_iface;
+};
+
+G_DEFINE_INTERFACE (GtkConstraintTarget, gtk_constraint_target, G_TYPE_OBJECT)
+
+static void
+gtk_constraint_target_default_init (GtkConstraintTargetInterface *iface)
+{
+}
diff --git a/gtk/gtkconstraint.h b/gtk/gtkconstraint.h
index 1ca9fd7f5e..1467c86d9c 100644
--- a/gtk/gtkconstraint.h
+++ b/gtk/gtkconstraint.h
@@ -24,6 +24,21 @@
 
 G_BEGIN_DECLS
 
+typedef struct _GtkConstraintTarget GtkConstraintTarget;
+
+#define GTK_TYPE_CONSTRAINT_TARGET (gtk_constraint_target_get_type ())
+
+/**
+ * GtkConstraintTarget:
+ *
+ * The GtkConstraintTarget interface is implemented by objects that
+ * can be used as source or target in #GtkConstraints. Besides
+ * #GtkWidget, it is also implemented by #GtkConstraintGuide.
+ */
+
+GDK_AVAILABLE_IN_ALL
+G_DECLARE_INTERFACE (GtkConstraintTarget, gtk_constraint_target, GTK, CONSTRAINT_TARGET, GObject)
+
 #define GTK_TYPE_CONSTRAINT (gtk_constraint_get_type ())
 
 /**