make the right-click graph selection interface on the left pane modal

[kohlhase]

Very often the various "graph types" do not apply to the current selection in the hierarchy. It would be good if they could be grayed out rather than just not doing anything.

[Shadow992]

As far as I know this is not too easy to implement at server? @Jazzpirate tried to explain me why this is not straight-forward to implement, but I forgot it.

[kohlhase]

then this issue would be a good place to document that.