app, libgimp*, plug-ins, icons: revert icon names into freedesktop...

... standard icon names and GTK+ icon names as second choice.
We should only use GIMP specific icon names as last resort, when there
is no standard or GTK+ names dedicated to the function.
This is made possible thanks to commit 3cc77b0.

s/gimp-document-recent/document-open-recent/
s/gimp-indent/format-indent-more/
s/gimp-next/go-next/
s/gimp-previous/go-previous/
s/gimp-save/document-save/
s/gimp-save-as/document-save-as/
s/gimp-revert/document-revert/
s/gimp-open/document-open/
s/gimp-document-recent/document-open-recent/
s/gimp-quit/window-close/ ou s/gimp-quit/application-exit/
s/gimp-warning/dialog-warning/
s/gimp-edit-clear/edit-clear/

s/gimp-justify-.*/gtk-justify-.*/
s/gimp-font/gtk-select-font/
s/gimp-color-palette/gtk-select-color/
s/gimp-cancel/gtk-cancel/

app/actions/gradients-actions.c

@@ -56,7 +56,7 @@ static const GimpActionEntry gradients_actions[] =
     G_CALLBACK (data_duplicate_cmd_callback),
     GIMP_HELP_GRADIENT_DUPLICATE },
 
-  { "gradients-copy-location", GIMP_STOCK_CLIPBOARD,
+  { "gradients-copy-location", "edit-copy",
     NC_("gradients-action", "Copy Gradient _Location"), NULL,
     NC_("gradients-action", "Copy gradient file location to clipboard"),
     G_CALLBACK (data_copy_location_cmd_callback),
@@ -68,7 +68,7 @@ static const GimpActionEntry gradients_actions[] =
     G_CALLBACK (data_show_in_file_manager_cmd_callback),
     GIMP_HELP_GRADIENT_SHOW_IN_FILE_MANAGER },
 
-  { "gradients-save-as-pov", GIMP_STOCK_SAVE_AS,
+  { "gradients-save-as-pov", "document-save-as",
     NC_("gradients-action", "Save as _POV-Ray..."), NULL,
     NC_("gradients-action", "Save gradient as POV-Ray"),
     G_CALLBACK (gradients_save_as_pov_ray_cmd_callback),