Listener for tab close

[hrj]

It should be possible to register a listener than gets called whenever a tab is about to be closed. And, it should be possible to cancel the closure through the listener.

Use cases

1. a tab with child tabs is being closed
   1. is collapsed -- could close only self or all children as well (possibly after confirmation)
   2. is not collapsed -- close only self if normal click, but close children also if Shift+Click, for example.
2. a tab has dirty content
   a tab could have unsaved changes, and hence require confirmation before closing.

To cover all these use-cases, various contexts need to be available to the listener:

1. tab is collapsed or not
2. state of keyboard modifiers like SHIFT.