adapter: Disconnect devices during power down

When the adapter is powered down, make sure all devices are
disconnected.
adapter: Add signal to announce powering down

Send a signal before the power-down procedure begins.
doc: Add adapter's PoweringDown D-Bus signal

An external component might need to distinguish device disconnections
from adapter being powered down (e.g. Bluetooth turned off). Unless a
specific signal is sent in the beginning of the procedure, the client
will first receive a profile state update (e.g. HandsfreeGateway state
set to 'disconnected') and therefore cannot realize what's happening.