Allow granting feature permissions permanently

In Chrome, if a site was previously allowed to use media devices (audio, video,...) the browser doesn't ask again the permission.

As a concequence enumerateDevice is correctly fill and no additionnal request to getUserMedia are necessary.

Under QWebEngine, the permission is not stored, as a concequence the featurePermissionRequested method is call, and a getUserMedia request is needed to get a full enumerateDevice.

-> getUserMedia implies to open media devices, and camera and audio devices are opened (noise could be hear on speaker, camera led is on).

==> could you allow to manage a list of already allowed origins in order to reduce the usage of getusermedia, the API doens't need to add origin, just use it and list and delete, this seems to be done on WebProcess side.