New session is appended to existing one ignoring new session panels structure

[iromeo]

The new session is appended to existing one w/o panels. PR (https://github.com/JetBrains-Research/epigenome/pull/1530) adds panels serialization into Session file (#248).

If some tracks are already shown in JBR, JBR suggests appending a new session to the existing one. If the user accepts suggestion (press No) and allow to append his multi-panel session, all tracks will be added to the last panel. Seems better is to

• If both sessions (existing and new) have only default panel - then keep current behaviour
• If one of session is multi-panel, then add new session as addition panels after panels of existing session.

[Xewar313]

I am not entirely convinced if adding panels to the end of session is a good idea - let's assume that we have a session with 3 panels, and a second session with additional 3 panels. That would result in a session with 6 panels, and these panels would be quite small, and almost unusable.
Are you sure we want to implement it that way?

[iromeo]

Agree, need to brainstorm a bit more