Disable QDebug (and QAssert by extension) for release builds

We should be making release builds without QDebug enabled so that Q_ASSERTs aren't bringing Mixxx down for users and so that user's log files are easier to read. This would also allow us to have more verbosity automatically in developer builds and saves the work of having to remove a bunch of qDebugs before a release.

In order to achieve this, we first need an easy way to log informational messages. Right now, we use qDebug for both informational messages the user can/should see as well as actual debug messages useful only to developers. I'm surprised that Qt doesn't have a similar qInfo function but I propose we create one in Mixxx that's about as easy to use as qDebug is.