Rewrite QxtLogger object to use Targets instead of Levels.

[mschnee]

We've moving from using static Log Levels to dynamic, user-customize-able Log Targets.

1. Retain the old methods for compatibility: qxtLog->info("message"); should send the message to the "INFO" target.
2. Update the workers that actually dispatch messages: this mostly means converting from LogLevel to QByteArray.
3. There are a lot of functions that don't make sense: functions that enable higher or lower log levels, for example. Consider new options and controls for the automatic creation of targets and default dispatch to log engines.