Consecutive ForkList Instructions should be supported

Current Situation:

• When a user tries to add two ForkList Instructions following each other, then the error is raised:

  Invalid target - Forklist instruction cannot be nested

Steps to reproduce:

• Create a workflow w1 and add a ForkList Instruction f1 with a job j1.
• Add a job j2 after ForkList Instruction f1.
• Add another ForkList Instruction f2 with job j3 after job j2
• Drag ForkList Instruction f2 and try to drop it after ForkList Instruction f1. 

Desired Behaviour:

• Consecutive ForkList Instructions should be supported.
• An error is raised for nestingn of ForkList Instructions only, if a user tries to add a ForkList Instruction inside an existing ForkList Instruction.