QtWidgets: use nullary version of qRegisterMetaType<T>("T")

QtWidgets: use nullary version of qRegisterMetaType<T>("T")

Using qRegisterMetaType<T>() has the advantage that multiple calls
during a program run are much more efficient, since an inlined
atomic is used to store the result. It also ensures that
Q_DECLARE_METATYPE(T) has been used, whereas qRegisterMetaType<T>("T")
will happily register anything.

Had to add Q_DECLARE_METATYPE to QFileInfo, for
QList<QPair<QString,QFileInfo>> of QFileSystemModel to work with
the partial specialisations of Q_DECLARE_METATYPE for QList, QPair.

In order to synchronize this change with other modules that did
their own Q_DECLARE_METATYPE(QFileInfo), a sync macro is defined
that can be tested in other modules, and will later be removed again.

Change-Id: I3004664e07e64cd885d5a03a57ff4e4379804aec
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>