Permissions: respect context object's thread and life time

When a context object is provided, then callers expect that the functor
or slot is executed in the thread of the context object. And if the
context object has been destroyed by the time the permission response
is received, the functor shouldn't be called at all.

To implement this, we either have to plumb the call back through a
signal/slot connection and benefit from QObject's infrastructure. This
is not practical here, as we don't have an "engine QObject" that would
emit a signal.

Instead, we can create a QMetaCallEvent explicitly, following what we do
in e.g. QHostInfo, and using a temporary QObject that handles the event
to then call the functor.

Add test coverage.

Pick-to: 6.5
Change-Id: Id878e45b304857304165ab4a7c6aae76fbee46ce
Reviewed-by: Fabian Kosmale <[email protected]>
Reviewed-by: Qt CI Bot <[email protected]>

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