As a user, I can see and manage Test Executions of Test Plans using Structure

As a user, I can see and manage Test Executions of Test Plans using Structure, so that Test Executions appear under the related Test Plan(s).

ACs:

• When adding Test Executions underneath a Test Plan, the Test Executions will be linked to the Test Plan and the Tests Execution's Tests will also be added to the Test Plan if they're not already there.
• When removing Test Executions that are underneath a Test Plan, the Test Executions will be unlinked/removed from the Test Plan; the Tests on the Test Plan shall not be affected by this operation, unless otherwise specified by XRAY-2955
• A flag shall be provided in Xray extender to enable this behaviour (unchecked, by default): "Manage Test Executions under Test Plans".