app: if there is no "text", generate the layer name from "markup"

Add gimp_markup_extract_text() which does just what it says (includes
code stolen from gmarkup.c), and use it if the layer's text object
doesn't have any text set.

app/core/gimp-utils.h

@@ -67,6 +67,9 @@ GimpObject * gimp_container_get_neighbor_of_active (GimpContainer   *container,
                                                     GimpContext     *context,
                                                     GimpObject      *active);
 
+gchar      * gimp_markup_extract_text              (const gchar     *markup);
+
+
 /* Common values for the n_snap_lines parameter of
  * gimp_constrain_line.
  */