Send dbus signal if powerkey is pressed during alarm / incoming call

Instead of completely ignoring the power key while there are active
alarms, mce now sends a dbus signal that alarm ui can react to and
for example snooze the alarm.

Similarly mce itself now ignores powerkey during incoming call, but
does send a dbus signal that call ui can react to and for example
silence the ringing.

[mce] Send dbus signal if powerkey is pressed during alarm / incoming call. Fixes JB#17794
Merge pull request #147 from spiiroin/powerkey_signal

Send dbus signal if powerkey is pressed during alarm / incoming call