QUPowerInterface: Do away with use of QDBusInterface.

This requires a blocking DBus call to set up, so even though it's a little more
verbose to do it by hand, it's worth it.

Change-Id: Iccf264658a870b554e73bc817b4e9372a3adae83
QUPowerDeviceInterface: Make every property request except the first one asynchronous.

Fetching UPower properties can take a long time (I've seen >20ms on a device
that isn't even under load), so minimizing the amount of time we wait on the
main thread for the reply makes sense.

We keep the properties we had before around, and only emit changed signals when
the properties actually change.

Change-Id: Ia3a111d16bf0149a9aa153bf08f37409663ff7be
QUPowerDeviceInterface: Remove getProperty helper

Just call QMap::value directly.

Change-Id: Ic108b0585892986448ea7707b6cf7f54502e7e02