Handle Virtual parent export type.

A "Virtual" export type is created in the metadata to have similar DOM when importing-exporting between QML and PSD.

AssetImported plugin has to handle this virtual group layer.

Changes:

• Make such layers as locked so that it can't be deleted accidentally. (Update: Locking is not a good idea. This disables the edit also)
• Handle Virtual export type in Qt-Bridge.
  • The export type of virtual parent layers should be disabled.
  • The immediate child can not be set as merged.
• Add child layers of virtual parent layers to their appropriate parent while exporting metadata in Qt-Bridge