As a user, I can define a Test Plan as including other Test Plans

As a user, I can define a Test Plan as including other Test Plans, so that it's possible to have an overview of multiple related Test Plans.

Thus, changes made in the "sub" Test Plans can be analyzed at the parent Test Plan.

Examples:

• adding a Test to a sub Test Plan will automatically add the Test to the parent Test Plan
• adding a Test Execution to a sub Test Plan will also reflect on the parent Test Plan