Before, when entering the media viewer, the MediaViewer struct was only
temporary, whereas the Data struct was kept alive. But the Data struct
was kept forever, even when exiting the media viewer. As a result,
the last media widget, i.e. Image or VideoPlayerWidget, seen in the
media viewer wasn't dropped either.
With this commit, the MediaViewer struct is kept as long as the
media viewer is open; the Data struct is kept with it. When the
user navigates out of the media viewer, both of them get dropped.
Therefore, the corresponding widget gets dropped, too.
4dfa5989f2