Offer Thunar specific file selection dialog via DBus Service "org.freedesktop.portal.FileChooser"
So that all applications which want to pick a file, will see the thunar dialog, instead of the foreign GtkFileChooserDialog.
The reason to offer such a dialog/service in thunar would be, that the 'known stuff' could be used, no need to get used to the different ui/ux of GtkFileChooserDialog.
I would not aim to support the full portal interface. Just this single service for now (Not a problem for thunar, since it anyhow runs as daemon)
The main thing to consider is, that existing widgets should be re-used as much as possible.Probably ThunarWindow could be extended/used as parent widget to inherit from. Or possible we need a abstract base-class and ThunarFileChooserDialog and ThunarWIndow inherit from that.
For more info, check the https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/64