Allow showDiff() to be used in other hosting apps

This patch brings the DiffWidget to other hosting apps. That does require two further additions to the KTextEditor::MainWindow interface, although those should again be trivial to implement if a hosting app already has an implementation for addWidget().

It may be debatable whether, instead, showDiff() should dispatch to a host-specific implementation. However, writing a (full) implementation of showDiff() would require a significant amount of code on the side of the hosting app. Should we get to the point where some plugins are developed outside of kate, and require showDiff(), this could still be revised, easily.