Build shared code for gphoto2 and mtp as a convenience lib

Deduplicate code between gphoto2 and mtp by creating a convenience
library call libgvfscommon-gphoto2 which is only built if either libmtp
is used or gphoto2 with gudev is used.

https://bugzilla.gnome.org/show_bug.cgi?id=736285
mtp: Improve root dir name and icon handling

If the device has no friendly name, set the display name of the device
from the udev info to match the name shown by the volume monitor.

This is visible as the folder title when the root dir of the device is
open in Nautilus (or using gvfs-info).

https://bugzilla.gnome.org/show_bug.cgi?id=736285
gphoto2: Improve root dir name and icon handling

Set the name of the root dir and the icon to match the way that the
volume monitor sets them.

This is visible as the folder title when the root dir of the device is
open in Nautilus (or using gvfs-info).

https://bugzilla.gnome.org/show_bug.cgi?id=736285
gphoto2: Add symbolic icon for volume

Add a symbolic icon to the volume using the new shared API.

https://bugzilla.gnome.org/show_bug.cgi?id=736285