app-chooser-dialog: make sure to hold a ref when doing async ops

app-chooser-dialog: make sure to hold a ref when doing async ops

Hold a ref to the GtkDialog while doing async operations with the
GtkAppChooserOnline object.
This is needed, since somebody could call gtk_widget_destroy() on us
while an async operation is in progress. We don't want to be finalized
in that case, but mark the fact that we were dismissed and just return
from the callback in that case.
This avoids crashing if the dialog is destroyed in the middle of a PK
operation.

https://bugzilla.gnome.org/show_bug.cgi?id=649121