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/designer/formwindoweditor.cpp

@@ -257,9 +257,9 @@ TextEditor::PlainTextEditorEditable *FormWindowEditor::textEditable()
     return &d->m_textEditable;
 }
 
-QString FormWindowEditor::preferredMode() const
+QString FormWindowEditor::preferredModeType() const
 {
-    return QLatin1String(Core::Constants::MODE_DESIGN);
+    return QLatin1String(Core::Constants::MODE_DESIGN_TYPE);
 }
 
 } // namespace Designer