Allow blanking via lpm when powerkey is pressed

By default pressing power key now goes to lpm mode before really
powering off the display.

The default can be changed via:
  mcetool --set-powerkey-blanking=off|lpm

[mce] Allow blanking via lpm when powerkey is pressed
Timer based blanking from lock screen happens via lpm state

If blanking timer triggers when display is fully powered up and
tklock is active, the display is put to lpm state for a while before
powering it of for real.

[mce] Timer based blanking from lock screen happens via lpm state
Implement dbus method for requesting display lpm state

The method is defined/documented in mce-headers 1.13.0. It allows
UI applications to request display blanking via low power mode state.

[mce] Implement dbus method for requesting display lpm state. Fixes JB#20906
Treat display off dbus requests as display lpm requests

This is transitional debugging feature and should be reverted once
lipstick uses the lpm mode request with swipe-down-to-lock gesture.
Do not change built-in default behavior of power key

Making powerkey press go via lpm state before actually turning off
the display requires now either a config file entry or opt in by
the user (via mcetool --set-powerkey-blanking=lpm).
Merge pull request #196 from spiiroin/lpm_enablers

Add enablers for entering lpm state, v2