EditorManager: Change openEditor/activateEditor to not do mode switching

Also introduce a indirection, modes have now types, edit and debug have
the same type, and editors do have a prefered type of modes instead of a
prefered mode.

That fixes the bug that if the prefered qml editing mode was set to
design, then in switching between qml editors would
- in Edit Mode: stay in edit mode
- in Debug Mode: switch to debug mode

src/plugins/designer/formeditorw.cpp

@@ -903,7 +903,7 @@ void FormEditorW::switchSourceForm()
     Core::EditorManager *em = Core::EditorManager::instance();
     const QString fileToOpen = otherFile(em);
     if (!fileToOpen.isEmpty())
-        em->openEditor(fileToOpen);
+        em->openEditor(fileToOpen, QString(), Core::EditorManager::ModeSwitch);
 }
 
 } // namespace Internal