Add option for disabling power key when proximity is covered

Similarly to double tap, also power key actions can now be
* always allowed
* never allowed
* allowed if proximity sensor is not covered

Default is "always", i.e. power key works the same way as before.

The setting can be changed via mcetool option
  -Z, --set-powerkey-action=<never|always|proximity>

The setting persists over mce / device restarts.

[mce] Add option for disabling power key when proximity is covered
Forget orientation state when sensor is stopped / sensord is not available

Allows logic elsewhere in mce to detect orientation sensor downtime.
Evaluate orientation sensor enable state on mce startup

If mce startup ends up using display on state, there are no display
state transitions that would trigger starting of orientation sensor.
Thus we need explicit orientation sensor start/stop evaluation
during mce startup.
Start orientation sensor already when powering up the display

This way display power up and sensor ramp up happen in parallel and
we should already have up to date sensor data when the display state
transition is finished.

This needs to be revisited when LPM_ON and LPM_OFF display states are
taken into use to avoid unnecessary sensor starts and stops.
Detect and broadcast device flip over gestures

If the device orientation changes from face up to face down while
the display is on and there is active call or alarm, an appropriate
ui feedback signal is sent over D-Bus System Bus.

[mce] Detect and broadcast device flip over gestures. Fixes JB#17820
Merge pull request #140 from spiiroin/powerkey_proximity

Add option for disabling power key when proximity is covered
Merge pull request #145 from spiiroin/detect_flipover

Detect and broadcast flip over gestures