Add self-contradiction filter panel

Added the self-contradiction filter panel to the SidebarPanel.
Self-contradiction panel: You can choose the desired statement-type, then the corresponding variables as well as the "agreement"-boolean. A Jtree will show the chosen variables for which a (self-)contradiction was found. By clicking on the statements involved, the statements are loaded from the documents.
TODO: Trimming and implementation of fail-safes needed.
Why such a strange name with "2" at the end? Why do we need this function at all? It seems to be a duplicate of a function that is already there. So why not use getVariableStringEntry instead?