wizard-source: Drop use of file chooser dialog
wizard-source: Drop use of file chooser dialog

Instead of creating and launching a separate dialog to let user select
ISO960 files/devices, use the new WizardWindow API to present an in-view
file chooser.

Nested dialogs is something we want to avoid according to our designers.

https://bugzilla.gnome.org/show_bug.cgi?id=741046

vbenes/gnome-boxes,vbenes/gnome-boxes