Replace QLoggingCategory::isEnabled by non-template functions

This yields the same results as previously and is more in line
with existing interfaces.

Change-Id: I0bf0372bf18f3bfde579385cddbe594bf71e3c52
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>