power: tidy code

* use initializeLibs/finalizeLibs to create/destroy
    libupower
    liblogin1
    libkeybinding
    libnotifications
    libsound
  and this tow methods will be invoked in Start/Stop.

* split Power.notifier and Power.player to independent function

* improve Stop method to handle module quiting

* handle_plan.go: there is no need invoke display.DestroyDisplay
  if the dbus object hasn't connected any signals.
  Cache the monitor's outputs.

Change-Id: I39d9e42b8141c3263a1df1ad5cf5da852922e71d