QSignalSpy: assorted improvements

QSignalSpy: assorted improvements

- Add const
- Add explicit (only effect under C++11;
  C++98 knows no multi-arg implicit conversions)
- Add Q_DECL_OVERRIDE to qt_metacall reimplementation
- Add reserve() calls on containers
- Change 'args' from QList to QVector for a 2x
  memory reduction on 64bit platforms

Change-Id: If6c779dac0499d4ab64e5191d37596edf7341f76
Reviewed-by: hjk <qthjk@ovi.com>
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>