FileChooser: Fix deprecation warnings
This fixes two deprecation warnings in the FileChooserDialog.
- PyGIDeprecationWarning: GObject.GError is deprecated; use GLib.GError instead
- Deprecation in button initialization, see: https://wiki.gnome.org/action/show/Projects/PyGObject/InitializerDeprecations?action=show&redirect=PyGObject%2FInitializerDeprecations
- Use of stock buttons is discouraged, I added 'Cancel' and 'Open' (with suggested-action class).
I don't know if this has any implications on Windows. Please test!