QtLogger: Implement categorized logging

Qtopia provided categorized logging as a better alternative to having random qDebug statements strewn throughout the code. The feature allowed groups of related diagnostic messages to be selectively switched on at runtime, thus allowing the user to see only the diagnostic data they are interested in rather than having the few meaningful messages swamped by all the other diagnostic/debugging output.

This approach has two further advantages: it assists maintenance (because developers can more readily see how the diagnostic code fits into the bigger picture) and it enables the Log-Based Testing methodology.

Investigate whether a similar approach to qLog can be provided in Qt5.