Session renaming

[philrz]

When the Sessions feature was first envisioned in #2467, it was proposed there should be a way to rename them. Per the implementation summary in #3132, right now a session's name is automatically taken from the text text of the most recent query in that session's history. Since the left panel is typically not very wide, and queries often start out similarly (from MyPool | ...) being able to assign unique names to disambiguate them may indeed be helpful.