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/qmljseditor/qmljseditor.h

@@ -81,7 +81,7 @@ public:
     QString id() const;
     bool isTemporary() const { return false; }
     virtual bool open(const QString & fileName);
-    virtual QString preferredMode() const;
+    virtual QString preferredModeType() const;
 
 private:
     Core::Context m_context;