BACKPORT: QUPowerInterface: dead code removal.

changed() signal was only connected to a completely empty handler.
onBattery and getProperty are also unused, only called by the commented-out
bits removed here.

getProperty also had a fun memory leak/efficiency issue (always creating a new
QDBusInterface, which always involves blocking calls on creation,
and never freeing it)

Change-Id: I597f52d152afb2d605698cab48d3b8a916ddf33e
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
BACKPORT: 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
BACKPORT: 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
BACKPORT: QUPowerDeviceInterface: Remove getProperty helper

Just call QMap::value directly.

Change-Id: Ic108b0585892986448ea7707b6cf7f54502e7e02
BACKPORT: Use QDBusReply instead of manipulating the QDBusMessage by hand.

Not only is this neater, it fixes a screwup in
1be686159ce23a4fc683b1c17926f923ff957513. QVariant apparently doesn't want to
implicitly convert the QVariantMap via toMap, but instead silently returns an
empty map.

Change-Id: I0a71bf54a0d69a7f2da6a26fa77c53f7d3d9a588
BACKPORT: QUPowerDeviceInterface: Fix a memory leak

Change-Id: I5e671429dcfd2968ccf5153601b0e8855cf0482b
Done-by: Mohammed Sameer <mohammed.hassan@jolla.com>