Basic declarative (QML) interface for Solid

It registers a class (org.kde.solid.)Devices (initially modelled after
DeviceNotifier) that contains the following features:
- ability to list all devices or just those that match a defined query
(Solid::Predicate)
- signals for when devices that match the query are ad->ed or
deleted
- isEmpty property - are there no devices that match the query?
- count property - the number of devices that match the query
- devices property - a list of udis of the devices that match
the query
- device method that returns a QObject interface to the device.

REVIEW:111964