Allow zero open sessions instead of forcibly opening a session on root

[idofrizler]

Keep the "generic" widgets and components that are not tied to a specific session (new session, session history, settings, some canvas with Cooper logo in the middle but with no conversation, or session specific attributes. you can keep the skeleton, but don't allow using those if no session is alive.

this is good both for new installs as well as when closing the last opened session (today's behavior of opening a placeholder session is weird).