Feature/keyboard interactions for menubutton and menu

[sjarva]

Modify the keyboard interaction for menu button and the menu according to WAI ARIA practices 1.1 for navigation menu

If menu button has focus, and

• down arrow, space or enter is pressed, menu opens and moves focus to first menu item
• up arrow is pressed, menu opens and moves focus to last menu item

While the menu is open, and

• enter is pressed: menu closes, menu button gets focus and the link inside menu item activates
• escape is pressed: menu closes, menu button gets focus
• up arrow is pressed: previous menu item focuses. (If focus is on the first menu item, cross button focuses. If cross button has focus, last menu item focuses.)
• down arrow is pressed: next menu item focuses. (If focus is on the last menu item, cross button focuses. If cross button has focus, first menu item focuses.)
• home is pressed: first menu item focuses
• end is pressed: last menu item focuses

What is not according to WAI ARIA practices

• tab is pressed (practices don't mention): menu closes and menu button focuses
• pressing keys from a to z doesn't focus on the menu item that starts with the letter in question. I'll create an issue about this.

[sjarva]

Created #410 to document what is still missing from recommended practices.

[sjarva]

And same goes for this one, if @negomi you feel that this PR is really contributing to this repository, I'd appreciate if you could label it as hacktoberfest-accepted, because I'm participating in Hacktoberfest (and they've changed the participation rules).

[negomi]

Very excited for these changes 🎉

Fixes #404 Fixes #130