"Start Root Mode Krusader" feature didn't work for me out of the box, turned out that it relies on kdesu tool. This tool doesn't appear to be available for modern Ubuntu distributions but kdesudo will work just fine. So this change will make sure that both kdesu and kdesudo are detected and configured if present. It also documents them as optional dependencies.

While at it, I've also added kde-cli-tools as an optional dependency. This is actually a transitive dependency - it contains the mime type editor required by the File Properties dialog provided by KF5::WidgetsAddons.

The other changes in the INSTALL dialog are merely trailing whitespace being removed (my editor does that automatically).