Allow setting of categorized logging rules without recompiling

Qt supports logging by categories (qCDebug ...) since 5.2. Individual categories and -levels can be activated/deactivated by text based logging rules. However, setting the rules is right now only possible through a method call:

http://doc-snapshot.qt-project.org/qdoc/qloggingcategory.html#setFilterRules

Any Qt application should be able to pick up rules from well known paths/environment, without having to add logic to the application itself.

See also: http://community.kde.org/Frameworks/Porting_To_qCDebug