inspector: improve network graph appearance and interaction

* Use a simple and tech-y black/blue/white color scheme.
  (In future, we could perhaps offer a selection of color schemes...)

* Keep individual systems as narrow as possible - it improves overview.

* Replace the 3 dots that indicated possibility of expansion with an
  expand/collapse button.
CMake: support older versions
inspector: RealvecWidget: fix destruction
inspector: implement creating new signal views
inspector: use dock widget title for control/system path
inspector: do not clear signal view when non-realvec control clicked
unit tests: fix paths to compiled executables
black box tests: fix peakClustering peak output test

* Fix path to peakClustering executable - get it from the build system.
* Let peakClustering use CMake's current binary dir as working dir.