The default navigation method should be TabbedNavigation

TabbedNavigation is when we have a MainWindow, i.e. the typical use case;
whereas NoNavigation is when using Konsole Part. The code in Part calls
setNavigationMethod(NoNavigation), so things should work as before.

I made a wrong assumption that TabbedNavigation was already the default.

CCBUG: 432077

src/ViewManager.cpp

@@ -52,7 +52,7 @@ ViewManager::ViewManager(QObject *parent, KActionCollection *collection) :
     _pluggedController(nullptr),
     _sessionMap(QHash<TerminalDisplay *, Session *>()),
     _actionCollection(collection),
-    _navigationMethod(NoNavigation),
+    _navigationMethod(TabbedNavigation),
     _navigationVisibility(NavigationNotSet),
     _managerId(0),
     _terminalDisplayHistoryIndex(-1)