Reparent file picker in fullscreen mode

[danxuliu]

• Requires Make possible to specify the container for the filepicker dialog nextcloud-libraries/nextcloud-dialogs#427

By default the file picker is a sibling of the fullscreen element, so it is not visible when in fullscreen mode. In #6879 the file picker is reparented shortly after opening it in fullscreen mode, but this hack should be removed once it is possible to specify the parent element of the file picker.