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/imode.h

@@ -52,6 +52,7 @@ public:
     virtual QIcon icon() const = 0;
     virtual int priority() const = 0;
     virtual QString id() const = 0;
+    virtual QString type() const = 0;
 
     void setEnabled(bool enabled);
     bool isEnabled() const;