gatt: Fix wrong device reference

gatt.c should keep only one reference of the device when the ATT
connection is established.
todo: Fix missing reply for Pair

Reports a bug that happens when Remove method gets called during the
pairing procedure. Pair reply is not returned.
gatt: Add GAttrib ref counting comment
gatt: Add unregistering GAttrib event handler

This patch unregisters the GAttrib callback responsible for managing
the ATT traffic when the connection is dropped and/or the device is
removed.
gatt: Fix changing service state when disconnecting

This patch notifies the "disconnected" state change for GATT based
services when disconnection happens.
proximity: Remove old Immediate Alert service
IAS: Move D-Bus signal emit code to a separate function
IAS: Only emit property changed D-Bus signal if alert value changes
IAS: Use error() for invalid characteristic value
IAS: Emit "none" alert level if monitor/locator writes invalid value
gatt: Use correct GATT write operation based on characteristic properties
gatt-service: Implement write support
FIXUP: gatt-service: Implement write support