Kill any running preview jobs before starting a new one

If loading a preview takes long, it was possible before this commit
that a preview for a new item was requested before the first preview
was shown. In that case, there was a race condition, and the first
preview to arrive stayed in the Information Panel.

This commit fixes this by keeping a pointer to the preview job and
killing it before starting a new one.

BUG: 250787
FIXED-IN: 4.12.1
REVIEW: 114561