Add 'leave.bs.tab' event

[mtarnovan]

If the event of leaving a tab needs to be captured, the only way to do this at the moment is to check relatedTarget on show/shown events. A typical use-case would be checking if a form contained in a tab has unsaved changes to show a warning to the user when we switches to another tab. So instead of code like this:

  $('a[data-toggle="tab"][href="#my-tab-with-form"]').one('leave.bs.tab', checkDirty)

we have to do something like this:

  $('a[data-toggle="tab"]').on('show.bs.tab', function (e) {
    if ($(e.relatedTarget).attr('href') == '#my-tab-with-form') {
      return checkDirty();
    }
  })

This also means that there's no way to only bind this once, because we emulate the leave event by checking the relatedTarget on show/shown.

[hnrch02]

hide and hidden would probably be more consistent names with all the other plugins' event names.

[mtarnovan]

Because the events trigger on the tab anchors, not on the tab-panes, I think hide/hidden would not be a good fit.

[hnrch02]

It's either show and hide or enter and leave. Doesn't make sense to mix them up.

[mtarnovan]

I see your point, and consistency across a framework is important, so I agree.

[cvrebert]

Fixed by #14772.