Refactor Results View for 2-level tree

[DenisKnoepfle]

Refactor the Results View, so that the main-tree contains only two levels.

The first level exists of problem names decorated with either a detected, not-detected or erroneous icon if an error has occurred during the diagnosis of this kind of problem. The second level will contain instances of occurrences of the corresponding problem type containing detailed information.

To implement this feature issue #61 needs to be solved first.