Replace log level DEBUG with INFO

[mixxxbot]

Reported by: uklotzde
Date: 2019-10-05T11:55:21Z
Status: Confirmed
Importance: Wishlist
Launchpad Issue: lp1846880

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The log level DEBUG = qDebug() is not intended to be enabled in production and may cause performance issues. This log level should only be enabled temporarily upon request or during development.

Instead, the new log level INFO = qInfo() (available since Qt5) should be used for writing detailed log messages into the log file. The default log level for console outputs should be WARNING = qWarning() as before.

This is a long-running migration that could not be completed instantly. At least all new code should follow these rules. The logging rules need to be documented in the developer guide.