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/coreplugin/modemanager.h

@@ -66,6 +66,8 @@ public:
     void addProjectSelector(QAction *action);
     void addWidget(QWidget *widget);
 
+    void activateModeType(const QString &type);
+
 signals:
     void currentModeAboutToChange(Core::IMode *mode);