Dropdown: close menu when focusing outside element

[pvdlg]

Currently the behavior of dropdown menu is inconsistent for mouse and keyboard users:

• when menu is open, clicking outside, close it
• when menu is open, focusing outside (tab key), doesn't close it

The changes in this PR close the dropdown menu when focusing on an element outside the dropdown menu.
It's handled in a similar fashion it's currently done for click events (except that focusin events inside the menu don't close it contrary to click events).

[twbs-rorschach]

Hi @vanduynslagerp!

Thanks for wanting to contribute to Bootstrap by sending this pull request!
Unfortunately, your pull request seems to have some problems:

• Changes must be made to the original Sass or Less source code file(s), not just the compiled CSS file(s).
• Changes must be made to the original JS source code file(s), not just the generated concatenated JS file(s).

You'll need to fix these mistakes and revise your pull request before we can proceed further.
Once you've fixed these problems, you can either ask the maintainers to re-open this pull request, or you can create a new pull request.
Thanks!

(Please note that this is a fully automated comment.)