...as touch is not exclusive to "mobile" anymore nowadays. also explicitly clarifies this is a fix for iOS, and that it impacts touch laptops etc as well. lastly, renames the variable from "dropdown" to "backdrop" for clarity/consistency

- Create backdrop only if the menu is actually open (do not create it if the show event is prevented)
- Drop the backdrop only when the corresponding menu is closed (do not remove if there is no menu to close or if the hide event is prevented)

* Close dropdown menu when focusing an outside element
* Update unit test to new markup

See https://travis-ci.org/twbs/bootstrap/jobs/179010884 for build failure

Dropdown perf - on keyboard nav, find active item in the active dropdown rather than the whole document (#19953)

Mostly KeyboardEvent.which and MouseEvent.which values.

[skip validator]

Dropdown: remove dependency to role="menu", role="listbox" a and li elements => fix keyboard navigation

Errata from:
* 8ce5da30
* 5d3144e8
which missed a few numbers.

[skip sauce]
[skip validator]

[skip sauce]
[skip validator]

[skip sauce]
[skip validator]

add modal my gawd