Another big deep optimization: eliminate the "Adding Level" step.
Before this change, everytime a level was checked, another level had to be created, i.e., by building the children of every link of the last checked level. This could take a *long* time.

Now, everytime a link is checked, a new ThreadWeaver job is created to build (in the background) the childs of that link, which will be a part of the next level.

Also drop ThreadWeaverGui because now there can be several Weaver instances and the maximum number of threads was augmented, so the result was a bad looking grid :)