This check tests that if a function has been declared deprecated
with either Q_DECL_DEPRECATED or has a @deprecated Doxygen note
then it MUST have both the Q_DECL_DEPRECATD and @deprecated note.
It's important that both are used, as Q_DECL_DEPRECATED allows
throwing a warning if that method is used in code, while the
@deprecated doxygen note gives an indication to devs/PyQGIS users
of why it's deprecated and what should be used instead.
Ideally we'd also test for SIP /Deprecated/ tags, but I can't
find any reliable way to do this.
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!