Count down for shut down dialog

Update count down label every second until the timer has expired.

Issue: #13
Power manager

Simple object that can power off, reboot, suspend, hibernate or sleep
the system.

It will be extended with different backends such as systemd and upower.
Power manager backend interface

Pure virtual base class for all power manager backends.

Issue: #13
systemd power backend

Backend for systemd.

Issue: #13
UPower power backend

Backend for UPower, yes I know there still machines without systemd.

Issue: #13
Static member that returns the DBus service name

This is handy if we need to know the DBus service we are using from
the PowerManager or elsewhere.

Issue: #13
Power manager backend name

This is convenient if we want to know how is the backend called from
the PowerManager or elsewhere.

Issue: #13
Load power manager backends

Implement actions and capabilities from our backends.
When services are registered/unregistered we load/unload the backends.

Issue: #13
Register PowerManager for QML

We are going to use it from the Shut Down dialog.

Issue: #13