Rename QMetaMethod::signature() to methodSignature()

Rename QMetaMethod::signature() to methodSignature()

In Qt5 the meta-data format will be changed to not store the
method signature string explicitly; the signature will be
reconstructed on demand from the method name and parameter type
information.

The QMetaMethod::signature() method returns a const char pointer.
Changing the return type to QByteArray can lead to silent bugs due to
the implicit conversion to char *. Even though it's a source-
incompatible change, it's therefore better to introduce a new
function, methodSignature(), and remove the old signature().

Task-number: QTBUG-24154
Change-Id: Ib3579dedd27a3c7c8914d5f1b231947be2cf4027
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>