investigate: backwards compatible session format

[toofar]

investigate: backwards compatible session format

[toofar]

This is resolved now, tree tab sessions are backwards compatible with the existing "flat" tab sessions and with the prior tree tab session format. One blocker down!

If anyone has been waiting for the churn around sessions to die down to switch from the old PR to the new one, now is the time.

@pinusc deftly switched the session format and code to re-use all of the existing session code with just a few small tree specific additions. I'm really happy to see an approach that aligns with the "how should it look like if we are maintaining it in the core codebase" mindset come together so fast.
There was a bit of cleanup required over the last couple of weeks but after @pylbrecht helped us get some tests setup for tree tab sessions it got wrapped up pretty quick.