Refactor UI

This removes the last "old-style" UI parts in the instrument editors (fixes #206 (closed)). A few spots still need a nicer layout, but after this MR this is only about working with QLayouts (more or less), not about migration the UI fundamentals any more.

Parts of the UI also are corrected regarding #175 (closed).

Also a few refactorings, simplifications and missing initializations have been added.

With these changes, a bunch of classes and files became obsolete.