app, libgimp, pdb: clean up some algorithm URLs.

- The "Healing Brush" paper URL is dead. It now leads to one of these
  horrible parasitic websites which reuse domain names. And we for sure
  don't want to promote this! I'm using instead the associated page on
  our developer website. I just created this page and right now it's on
  testing only. But this will be the URL for the main website.
- The URL for the foreground extraction paper is still valid, but from
  my small research today when writing various algorithm' pages, I think
  our own implementation diverged from this original paper (though it
  may still have part of this original paper's algorithm; only a deeper
  check of the code would tell).
  See: https://testing.developer.gimp.org/core/algorithm/foreground-extraction/
  Also this PDB procedure is called generically gimp_drawable_foreground_extract()
  and it even has an argument to choose the algorithm (even though right
  now, there is only a single choice, which is "Matting" algorithm, and
  even there you can't choose which one; it's always Matting Global;
  Matting Levin cannot be chosen for instance).
  So anyway it is not a good idea to point to one specific algorithm,
  and even less to leave an external URL (which may also disappear some
  day) in API docs. So I am just getting rid of this paper title and
  URL, and replace it with actually useful information, which is how to
  set the trimap to represent foreground, background and uncertain
  pixels (note: I notice that it would have been different with Matting
  Levin where uncertain pixels must apparently be set transparent).

app/paint/gimpheal.c

@@ -427,8 +427,8 @@ gimp_heal_laplace_loop (gfloat *pixels,
 
 /* Original Algorithm Design:
  *
- * T. Georgiev, "Photoshop Healing Brush: a Tool for Seamless Cloning
- * http://www.tgeorgiev.net/Photoshop_Healing.pdf
+ * T. Georgiev, "Photoshop Healing Brush: a Tool for Seamless Cloning"
+ * See: https://developer.gimp.org/core/algorithm/healing/
  */
 static void
 gimp_heal (GeglBuffer          *src_buffer,

app/pdb/drawable-cmds.c

@@ -2222,7 +2222,7 @@ register_drawable_procs (GimpPDB *pdb)
                                "gimp-drawable-foreground-extract");
   gimp_procedure_set_static_help (procedure,
                                   "Extract the foreground of a drawable using a given trimap.",
-                                  "Image Segmentation by Uniform Color Clustering, see https://www.inf.fu-berlin.de/inst/pubs/tr-b-05-07.pdf",
+                                  "This procedure extracts the foreground from @drawable using @mask as a trimap. Set white as foreground, black as background and uncertain pixels as any other value, for the tri-map.",
                                   NULL);
   gimp_procedure_set_static_attribution (procedure,
                                          "Gerald Friedland <fland@inf.fu-berlin.de>, Kristian Jantz <jantz@inf.fu-berlin.de>, Sven Neumann <sven@gimp.org>",

libgimp/gimpdrawable_pdb.c

@@ -1223,8 +1223,9 @@ _gimp_drawable_sub_thumbnail (GimpDrawable  *drawable,
  *
  * Extract the foreground of a drawable using a given trimap.
  *
- * Image Segmentation by Uniform Color Clustering, see
- * https://www.inf.fu-berlin.de/inst/pubs/tr-b-05-07.pdf
+ * This procedure extracts the foreground from @drawable using @mask as
+ * a trimap. Set white as foreground, black as background and uncertain
+ * pixels as any other value, for the tri-map.
  *
  * Returns: TRUE on success.
  *

pdb/groups/drawable.pdb

@@ -1201,8 +1201,9 @@ sub drawable_foreground_extract {
     $blurb = 'Extract the foreground of a drawable using a given trimap.';
 
     $help = <<'HELP';
-Image Segmentation by Uniform Color Clustering, see
-https://www.inf.fu-berlin.de/inst/pubs/tr-b-05-07.pdf
+This procedure extracts the foreground from @drawable using @mask as a trimap.
+Set white as foreground, black as background and uncertain pixels as
+any other value, for the tri-map.
 HELP
 
     $author    = 'Gerald Friedland <fland@inf.fu-berlin.de>, Kristian Jantz <jantz@inf.fu-berlin.de>, Sven Neumann <sven@gimp.org>';