Improve Logging system to be able to retrieve the registered logging categories

To interactively list logging categories one needs to have a list of available categories. A list at run time could be useful to change filters and logging rules while a program is running (for example through a console cli or webinterface). The data is already stored inside the QLoggingRegistry but it is not publicly available.

Additional information provided through this new class could be:

• a string representation of the QVector<QLoggingRule>
• a string representation of the QLoggingCategory::CategoryFilter