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 3cc77b03.

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/