Wt

In #13187 a change was introduced that wrongly made Wt::Http depend on main Wt. This cannot be allowed, and will result in potential build failures.
However, this is only the case when building with Wt, after it is installed, not something that is automatically tested.

As such to avoid such future issues, as this can be quickly read over, we should ensure a pipeline exists that not only builds Wt, but installs it, and uses it to make another project (e.g. widget gallery).