Create a common directory for extra image files, and move the video
thumbnails there. This folder structure is required to work around
the fact that relative image paths passed to qhp.<project>.extraFiles
are not resolved correctly when loading the generated help project.
Change-Id: Iff5583a9a318670571b7f706182723e54e0ec466
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
