CachingFieldScreenStore returns a mutable FieldScreen

CachingFieldScreenStore contains a cache of FieldScreen (which in turn contains a list of FieldScreenTab, which in turn contains a list of FieldScreenLayoutItem. This cache is returned as-is to callers of CachingFieldScreenStore, which exposes all sorts of concurrency bugs, such as:

• FieldScreen, FieldScreenTab, and FieldScreenLayoutItem can not be safely used across different threads
• clients of CachingFieldScreenStore can change the cached values directly by calling setters on any of the above classes.

We need to make a deep copy of the FieldScreen and its components before returning it to clients.