Add variant import through GSvar

[marc-sturm]

ToDo:

• [x] implement GSvarServer variant annotation interface which uses megSAP in the background (AC)
• [x] variant import
  • list variant (support different formats)
  • NGSD annotation from GSvarServer variant annotation interface
• [x] variant tab: allow showing annotations
  • use GSvarServer variant annotation interface
• [x] move other imports to import dialog