[NEW] Group conflicting statements

[sverhoeven]

Is your feature request related to a problem? Please describe.

I would like to visualize that a formal and informal statement are conflicting with each other.

Describe the solution you'd like

I would like to see red border around some formal and informal statement pair.
To do this I would like someway to select both statements and mark them as conflicting.

[sverhoeven]

in #141 instead of red border a red dotted edge is drawn between conflicting statements in both network pages.