Keep FileChooserNative alive while a portal is running

Keep FileChooserNative alive while a portal is running

Even if the FileChooserNative instance drops out on us while we're still
waiting for the portal to answer, we should keep the data and pointers
alive until the sequence of asynchronous operations is running. The code
already tries to do that, by acquiring a strong reference to the
GtkFileChooserNative instance, but it's also freeing data as soon as the
dialog is hidden, while asynchronous callbacks that will look at the
fields on that data are still in flight.

To avoid that, we defer freeing the data until the asynchronous
callbacks are invoked, and we keep a reference on the dialog while we're
emitting signals on it.

Fixes: #4883