app: Use G_DEFINE_BOXED_TYPE()

This isn't any functional change, but allows us to get rid of some of
the GObject boilerplate.

app/core/gimpbezierdesc.c

@@ -29,18 +29,7 @@
 #include "gimpboundary.h"
 
 
-GType
+G_DEFINE_BOXED_TYPE (GimpBezierDesc, gimp_bezier_desc, gimp_bezier_desc_copy, gimp_bezier_desc_free)
-gimp_bezier_desc_get_type (void)
-{
-  static GType type = 0;
-
-  if (! type)
-    type = g_boxed_type_register_static ("GimpBezierDesc",
-                                         (GBoxedCopyFunc) gimp_bezier_desc_copy,
-                                         (GBoxedFreeFunc) gimp_bezier_desc_free);
-
-  return type;
-}
 
 GimpBezierDesc *
 gimp_bezier_desc_new (cairo_path_data_t *data,

app/core/gimpdashpattern.c

@@ -30,18 +30,8 @@
 #include "gimpdashpattern.h"
 
 
-GType
+typedef GArray GimpDashPattern;
-gimp_dash_pattern_get_type (void)
+G_DEFINE_BOXED_TYPE (GimpDashPattern, gimp_dash_pattern, gimp_dash_pattern_copy, gimp_dash_pattern_free)
-{
-  static GType type = 0;
-
-  if (! type)
-    type = g_boxed_type_register_static ("GimpDashPattern",
-                                         (GBoxedCopyFunc) gimp_dash_pattern_copy,
-                                         (GBoxedFreeFunc) gimp_dash_pattern_free);
-
-  return type;
-}
 
 GArray *
 gimp_dash_pattern_new_from_preset (GimpDashPreset preset)

app/vectors/gimpanchor.c

@@ -27,18 +27,7 @@
 #include "gimpanchor.h"
 
 
-GType
+G_DEFINE_BOXED_TYPE (GimpAnchor, gimp_anchor, gimp_anchor_copy, gimp_anchor_free)
-gimp_anchor_get_type (void)
-{
-  static GType anchor_type = 0;
-
-  if (!anchor_type)
-    anchor_type = g_boxed_type_register_static ("GimpAnchor",
-                                                (GBoxedCopyFunc) gimp_anchor_copy,
-                                                (GBoxedFreeFunc) gimp_anchor_free);
-
-  return anchor_type;
-}
 
 GimpAnchor *
 gimp_anchor_new (GimpAnchorType    type,