feat(Dropdown): Single selection

[Silviu Alexandru Avram (silviuaavram)]

Single Selection Dropdown. Created with a Button instead if an Input.
Behaviours:

1. Button:

• on click, enter, space, should open the list and move focus to it. If item is already selected, that should be the one highlighted after opening (last part not working atm, added as improvement request).
• on up/down, should open the list and move focus to it. If no item is already selected, should focus on first or last item. If item is already selected, focus should be on the selected item position +/- 1. (last part not working atm, added as improvement request).

2. List:

• up/down should move focus through the options.
• home/end should move focus to first/last option (not working atm, improvement request).
• enter should select option and move the focus to the button.
• tab and shift tab should select option and move focus to next/previous element (button).
• click should select option and move focus to the button.
• selecting an option should trigger aria-live with ' has been selected.'
• letter key should focus on items starting with that key (not working atm, improvement request).

Navigation with Screen Readers should work as well, with and without virtual cursor.
Button should be made shorthand, in a future task.

Added some improvement comments in #560 while working on this.

[Marija Najdova (mnajdova)]

Some minor comments/opinion. The changes are going in the right direction :)

[Marija Najdova (mnajdova)]

👍 all issues are tracked, so we are save to merge this one!