Deprecate/Remove "emit" macro definition

Consider making the emit macro definition in qobjectdefs.h opt in, and removing it's use from the examples and code snippets in the documentation, replacing it with a [[qt::emit]] C++ attribute.

C++ 20 will most likely contain new classes with emit() member functions: wg21.link/P0053 . This means that including such C++ standard headers after qobjectdefs.h might lead to hard to understand compiler errors.