gvfs: rework and add recipe for gvfs-gdu-volume-monitor

To make gvfs work with removable devices/volumes, gvfs-gdu-volume-monitor
( gdu = gnome-disk-utility ) is required. During a build from scratch
gvfs-configure disables gdu because it can not find gnome-disk-utility.
It can not be added to gvfs DEPENDS because this causes circular dependencies:

nautilus DEPENDS gvfs
gvfs DEPENDS gnome-disk-utility
gnome-disk-utility DEPENDS nautilus

The solution is a new recipe for gvfs-gdu-volume-monitor. To reduce maintenance
efforts and to ensure that gvfs-gdu-volume-monitor is build under same
conditions as gvfs, as much settings as possible were moved to gvfs.inc. INC_PR
is started at 6 to ensure rebuilding.

run-tested for incremental build and build from scratch.

Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>