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/designmode.cpp

@@ -164,6 +164,11 @@ QString DesignMode::id() const
     return QLatin1String(Constants::MODE_DESIGN);
 }
 
+QString DesignMode::type() const
+{
+    return QLatin1String(Constants::MODE_DESIGN_TYPE);
+}
+
 QStringList DesignMode::registeredMimeTypes() const
 {
     QStringList rc;