Implement "/media::" default host name in Tramp

Implement "/media::" default host name in Tramp

* doc/misc/tramp.texi (GVFS-based methods): Describe default
/media:: file name.

* lisp/net/tramp-cache.el (tramp-get-file-property)
(tramp-set-file-property): Check, whether
`tramp-cache-{g,s}et-count-*' objects are numbers.

* lisp/net/tramp-gvfs.el (top): Don't set global default for
"media" in `tramp-default-host-alist'.
(tramp-gvfs-handler-volumeadded-volumeremoved): New defun.
(top): Register "org.gtk.Private.RemoteVolumeMonitor.VolumeAdded"
and "org.gtk.Private.RemoteVolumeMonitor.VolumeRemoved" signals.
(tramp-get-media-devices): Set defaults for "media" in
`tramp-default-host-alist'.