In a Qt application, the QApplication object should be the first QObject
that is created. However, the `Meta` class used to have a static member
called `mp`, which means that this member gets initialized _before_
main() runs and therefore before the QApplication is created.
This has caused an "Invalid nullptr in QObject::connect" warning
somewhere in Qt's internals (since the move to Qt 6). The impact of this
warning is unclear at this point.
This commit makes the mp parameter a std::unique_ptr that gets
explicitly initialized in the main function (more or less right after
the QApplication object is created). This guarantees that the MetaParams
object does not get created before the QApplication object, fixing the
observed warning.
It is worth noting that we do have a couple of other static QObject
variables in the main translation unit, but these seem to be
inconsequential (at least they don't seem to trigger a similar warning).
Fixes#6669
