enhancement: show window/session tab count in a window title

[ootada]

• suggested format: "session name|123|current tab title"
  (I'd favor no extra spacing to allow showing more of actual titles, but its debatable)
• (optional) add setting to customize window title format

additionally:

• this implies that every session lives in its own window
• its still currently possible to have tabs added to a session from a different window by context menu which IMHO breaks consistency and should be evaluated separately ( related: inconsistent behaviour of "Save tabs as session" #71 )

[tim-we]

Tabs Aside differentiates between windowed sessions and non-windowed sessions. Adding tabs to a windowed session from a different window should move them to that window, so I consider that a bug.