app: properly initialize property colors.

Our code expects that these GeglColor properties are always non-NULL in various
places, which may lead to crashes.

The property defaults should take care of this, and I am actually unsure why we
need to set it in the init(). Maybe the set_default() method for the GParamSpec
kicks in a bit late, while we have code syncing objects already running. I'm not
sure. It might need to be investigated later. But for now, let's simply make
sure we initialize the objects manually so that it's never NULL.

app/text/gimptext.c

@@ -399,6 +399,8 @@ gimp_text_class_init (GimpTextClass *klass)
 static void
 gimp_text_init (GimpText *text)
 {
+  text->color              = gegl_color_new ("black");
+  text->outline_foreground = gegl_color_new ("gray");
 }
 
 static void