The module juce_dsp requires C++14. You can enable it by selecting C++14 in the drop-down next to C++ Language Standard in the Project Settings in Projucer.
Maybe this could be a little bit optimized, instead a sign is much more intuitive.
(Which should be also shown in the parent folder and in the header bar, so that you also see it when the elements are folded.)
BTW: I found the tree-structure, where you click on the parent element much more intuitive than clicking on the wheel