Make ld_canvas_new() return a GtkWidget reference.

For consistency with GTK+.

liblogdiag/ld-canvas.c

@@ -560,7 +560,7 @@ update_adjustments (LdCanvas *self)
  *
  * Create an instance.
  */
-LdCanvas *
+GtkWidget *
 ld_canvas_new (void)
 {
 	return g_object_new (LD_TYPE_CANVAS, NULL);

liblogdiag/ld-canvas.h

@@ -66,7 +66,7 @@ struct _LdCanvasClass
 
 GType ld_canvas_get_type (void) G_GNUC_CONST;
 
-LdCanvas *ld_canvas_new (void);
+GtkWidget *ld_canvas_new (void);
 
 void ld_canvas_set_diagram (LdCanvas *self, LdDiagram *diagram);
 LdDiagram *ld_canvas_get_diagram (LdCanvas *self);