Fix context menu tabbing (#844)

* Tab compatible with Arrow key

* comment

* Refactor constants to have somewhat meaningful names
Rename some methods to be more related to what they do

* Restore catch-all logic that relates to layer context menu, but is 
obscure and complicated.  Use KeyboardEvent.code for its case-insensitive
mnemonics.

* Fix Firefox issue where Shift+F10 would always display the context
menu near the origin of the map container.

* Add tests for tab / shift+tab keyboard navigation of context menus

* Support Shift+Tab equivalent to ArrowUp in map context menu

* Support Shift+Tab equivalent to ArrowUp in layer context menu

* Prevent default on Arrow keys to prevent unwanted document scrolling

* Fix error when Shift+F10 / contextmenu pressed on features

* Fix firefox fix

* Simplify odd logic - let's see if tests pass...

* Test that error doesn't happen with Shift+F10 on features

* Test arrow keys on context menu don't trigger page scroll

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Co-authored-by: Jacky0299 <jackyissuperman@gmail.com>
Co-authored-by: prushfor <prushfor@L-BSC-A146390.nrn.nrcan.gc.ca>

Author: 3 people