Ideas to make ebp more accessible

[mydea]

Related to #1306, I think there are a few fundamental changes that should be made to make this addon more accessible. Currently, it fails e.g. ember-a11y-testing checks, sometimes in fundamental ways.

IMHO the most straightforward way to make the behavior of this addon more accessible would be:

1. Make the options in the single select a radio-button list
2. Move the search of the multi-select into the dropdown (same as in single select), to have no nested interactives in trigger
3. Make the options in multi select a list of checkboxes
4. Make the "x" remove functionality in multi-select trigger "optional" (to avoid nested interactives) - you can still remove it via the dropdown.

This is how this looks in our app, as reference:

With such a setup, I was able to satisfy ember-a11y-testing with a relatively simple setup. It also means that keyboard support for selecting/unselecting items etc. works pretty much out of the box without any special handling.

However, doing anything in this direction would definitely be a breaking change, so not sure if there is any interest in actually pursuing something like this. If so, I'd be happy to write up a PR.

[mkszepp]

Just as note.. we did nr. 2 in #1864 and is released in v8.6