app, libgimp*, modules, pdb, plug-ins: new GimpParamColor.

This is meant to obsolete GeglParamColor with at least an additional argument
has_alpha which we need in GIMP. It allows to advertize when a parameter wants
an opaque color, which in particular means we know when displaying a GUI to pick
colors with alpha or not.

app/dialogs/preferences-dialog-utils.c

@@ -231,15 +231,12 @@ prefs_color_button_add (GObject      *config,
 {
   GtkWidget  *button;
   GParamSpec *pspec;
-  gboolean    has_alpha = TRUE;
+  gboolean    has_alpha;
 
   pspec = g_object_class_find_property (G_OBJECT_GET_CLASS (config),
                                         property_name);
 
-  /* TODO: GimpRGB parameter specs had an "alpha" argument, unlike GeglColor
-   * space. We need to add back such an argument.
-   */
-  /*has_alpha = gimp_param_spec_rgb_has_alpha (pspec);*/
+  has_alpha = gimp_param_spec_color_has_alpha (pspec);
 
   button = gimp_prop_color_button_new (config, property_name,
                                        title,