[API] Store type information for methods in the meta-object

The meta-object data is currently storing type information for methods as strings (signature, parameters, return type). This makes introspection through QMetaMethod utterly inefficient; the signature is re-parsed again at runtime, type names are copied to QByteArrays, and the types have to be looked up by name (QMetaType::type()).

If the meta-type id of the return type and parameter types can be computed at meta-object definition time, they could be stored in the
meta-data for fast and easy retrieval. This is already (partially) supported for meta-properties (see QMetaProperty::userType()), but not for methods.

This change is being implemented at http://codereview.qt-project.org/#change,14763

Once this is done, we should also remove support for the old (Qt4) meta-data revisions (<= 6).