Implement set-file-* functions for tramp-gvfs.el

Implement set-file-* functions for tramp-gvfs.el

* lisp/net/tramp-gvfs.el (tramp-gvfs-gio-mapping):
Add "gvfs-set-attribute".
(tramp-gvfs-file-name-handler-alist):
Add `tramp-gvfs-handle-set-file-modes',
`tramp-gvfs-handle-set-file-times' and
`tramp-gvfs-handle-set-file-uid-gid'.
(tramp-gvfs-handle-set-file-modes)
(tramp-gvfs-handle-set-file-times)
(tramp-sh-handle-set-file-uid-gid): New defuns.

* lisp/net/tramp.el (tramp-handle-write-region): Set file modes.

* test/lisp/net/tramp-tests.el (tramp-test20-file-modes)
(tramp-test22-file-times): Do not skip for tramp-gvfs.el.