core: gatt: Implement GattCharacteristic1.StartNotify

This patch implements the StartNotify method of
org.bluez.GattCharacteristic1. Each call to StartNotify assigns a
session to the D-Bus sender which internally registers a unique
notify handler with the bt_gatt_client. Each received notification
or indication causes a PropertiesChanged signal to be emitted for the
"Value" property.

The notify handler gets automatically unregistered when sender
disconnects from D-Bus.
core: gatt: Implement GattCharacteristic1.StopNotify

This patch implements the StopNotify method of
org.bluez.GattCharacteristic1.