This is done only inside LFUN_BUFFER_VIEW_CACHE so as to preserve
the benefits of caching in other places.
Without this commit, if a converter or a user (re)moves the preview
file and then calls buffer-view-cache LyX will launch the viewer for
a file that does not exist.
For a discussion and use case, see:
https://www.mail-archive.com/search?l=mid&q=
55D393D7.
6050403@lyx.org
break;
case LFUN_BUFFER_VIEW_CACHE:
+ (d->preview_file_).refresh();
enable = (d->preview_file_).exists();
break;