gatt-dbus: Add remote GATT service objects.

This patch adds the initial GATT client D-Bus API support for remote primary
GATT services. Characteristics, descriptors, and included services are not yet
handled.
gatt-dbus: Add remote GATT characteristic objects.

This patch adds remote GATT characteristic objects to the D-Bus API. The
API only exposes the UUID; the characteristic value and permissions have
not yet been implemented.