gatt: Introduce helper for registering a notifier

This patch adds the btd_gatt_add_notifier() helper which registers a
notifier to be warned when the specified attribute's value changes.
gatt: Introduce helper for unregistering a notifier

This patch adds the btd_gatt_remove_notifier() helper which unregisters
a previously registered notifier for an specific attribute.
lib: Move GATT UUIDs to uuid.h
proximity: Add new Immediate Alert Service files

This patch adds new files to implement Immediate Alert Service using
the new attribute API. Files using the old GATT API will be removed
later. The proposed solution doesn't handle multiple local Bluetooth
adapters yet.
proximity: Add Immediate Alert Service

This patch adds the service and characteristic declaration of the
Immediate Alert Service using the new attribute API.
proximity: Add remove Immediate Alert Service

This patch removes the Immediate Alert Service declaration when the
adapter is removed. The current solution doesn't handle multiple local
Bluetooth adapters.
proximity: Remove unneeded control logic

This patch removes unneeded list of adapters and devices. device_remove
callback of the btd_profile always gets called when the device or the
adapter related to the device is removed.
scanparam: Remove unneeded header include
alert: Remove unneeded header include
gatt: Add listen socket for GATT server

This patch adds the initial code for the new GATT server integrated with
the Generic Attribute API. The legacy code will stay for a while.
gdbus: Add g_dbus_get_flags()
gatt: Enable new server if experimental is set

The current GATT server conflicts with the new Attribute Server that is
being developed. This patch enables the new Attribute Server is
Experimental option is informed.
gatt: Add GAP service

This patch adds GAP service definition and it's attributes in the new
Attribute server.
gatt: Add GATT service

This patch adds GATT service definition and it's attributes in the new
Attribute Server.
gatt: Add database dump

This patch adds an utility function to dump the Attribute Database.
gatt: Add attribute parameter to the read/write callbacks
gatt: Register ATT commands/events handler

This patch registers GAttrib callback to handle later ATT commands
requests and events.
gatt: Support ATT read request

This patch adds support for handling ATT read request PDUs.
FIXME: Investigate if there is another solution
gatt: Add Read By Group Type

This patch adds the initial ATT Read By Group Type operation. This ATT
operation is used by the GATT operation to Discover All Primary
Services.