Let Q_DECLARE_LOGGING_CATEGORY and Q_LOGGING_CATEGORY return a const reference

In general QLoggingCategory should be treated as a const object that's
configured by the backend. The only legitimate place where user code
should call setEnabled is in a CategoryFilter ...

[ChangeLog][QtCore][Logging] Make Q_LOGGING_CATEGORY
and Q_DECLARE_LOGGING_CATEGORY return a const object.

Change-Id: I6140442ab48286e05cd3b55064a502bbe6dfb16a
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>