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/projectexplorer/projecttreewidget.cpp

@@ -300,7 +300,7 @@ void ProjectTreeWidget::openItem(const QModelIndex &mainIndex)
     Node *node = m_model->nodeForIndex(mainIndex);
     if (node->nodeType() == FileNodeType) {
         Core::EditorManager *editorManager = Core::EditorManager::instance();
-        editorManager->openEditor(node->path());
+        editorManager->openEditor(node->path(), QString(), Core::EditorManager::ModeSwitch);
     }
 }