Add a driverHandle() method similar to the one in AudioInterface to PortableMediaPlayer and Camera.

Allows to get the information needed to access the device with a particular driver without having to rely on backend dependent data.

This way no need to expose low level related methods.

Jeff, in the hal backend I only implemented the support for the mtp driver, that will allow you to completely remove the use of GenericInterface in your ioslave. You'll have to add the support for the other drivers though. Thanks in advance.

WARNING: Please don't commit code depending on those new methods outside of kdelibs before coming monday.