Use category names when logging to the journal

Use the custom field QT_CATEGORY to store the name of the QLoggingCategory used
when writing to systemd's journal. To pass custom fields sd_journal_send() is
needed, and is used in combination with #define SD_JOURNAL_SUPPRESS_LOCATION to
store the metadata that is already in the QMessageLogContext.

Change-Id: I6a120701f7012aaa46451dd3d91586a419c5f803
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>