Improve the performance of test plan reference

Some users are heavily using TestPlanReference test steps. We should ensure the following scenarios are optimized with regards to performace:

Saving/Loading test plan reference plans. Multiple test steps could use the same test plan XML. This scenario should be possible to optimize. (FIXED BY !383 (merged))
Annotating a test plan reference step (!404 (merged))
Annotating multiple test plan reference steps. (!404 (merged))
Consider: Binary serialization or local change serialization (with annotations) for undo/redo functionality.

Edited May 18, 2020 by Asger Iversen