refactored active navTab in contentView for marking/highlighting the active viewtab. Enhanced navigation logic: Clicking on an already selected NavlistItem changes the tree plus one (click on manuscript switches to pages tree). If a selected item of the last tree is clicked, it navigates to the tln-viewer if manuscript viewer is active. Then a change of the NavTab now sets also the navContext if tln-search or tln-crossRef are routed/displayed. Therefore it could be switched between showing the crossrefs of a page or of the manuscript depending on navContext.