Switch to non-streaming version of qWarning()

Allows us to drop the qdebug.h include in a public and frequently used
header.

Change-Id: I2cb05fbec6298ba600020db684e9eea4093ce663
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>