Clarify parentId specification in EntryModel

[bhufmann]

Right now the parentId in the EntryModel is specified as optional and if not present it means that the Entry doesn't have a parent. Being optional has the advantage that it only has to be transported if there is a parent.

In the Trace Compass server reference implementation the parentId is always serialized and value -1 is used to indicate that there is no parent. The theia-trace-extension front-end was aligned to the undocumented special value and handling for value -1.

This tracker is to decide on the wanted specification for the parentId field.