tools: Add setting Value property of gatt-service
tools: Add setting Value property of gatt-service

This patch extends the gatt-service.c example adding a generic callback
to allow set the characteristic Value. It doesn't check for
characteristic properties yet.

mapfau/bluez,mapfau/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,mapfau/bluez,pkarasev3/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pkarasev3/bluez,silent-snowman/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez