Screensharing on wayland triggers multiple xdg portal dialogs

navigator.mediaDevices.getDisplayMedia triggers two portal dialogs on wayland one for screen selection and one for windows.
This makes the DesktopMediaRequest api annoying to use.
Users have to cancel these dialogs multiple times before using the actual qt specific api/dialog.
It can be reproduced with the webrtc manual test and simplebrowser.

Chromium shows/hides these portal dialogs based on the state of their own dialog's.
Firefox triggers only one portal dialog that shows both screens and windows and does not show its own dialog.

Proposed solutions:
 1. extend the DesktopMediaRequest api with show/hidePortalDialog so users can implement the chrome stlye dialog. It is easy to implement but can be complicated to use.
 2. do everything behind the scenes and provide a "generic" portal dialog same as firefox. It is harder to implement because we would need direct access to webrtc methods and currently the MediaRequest api is based on the DesktopMediaList structures which don't have this functionality.