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/editmode.h

@@ -58,6 +58,7 @@ public:
     int priority() const;
     QWidget* widget();
     QString id() const;
+    QString type() const;
     Context context() const;
 
 private slots: