Support using litehtml as help back end

We are hoping to be able to use the litehtml help viewer backend as default for displaying helps in Qt Creator for Qt 6, so that we can get better styling and more advanced help functionality. Therefore, it is crucial that Qt Design Studio also supports it.

In 1.6.0, if I set litehtml as help backend in Tools > Options > Help > Viewer backend, and restart QDS, the help view is blank.