fix(commons/dom): change isFocusable to check visibility in screenreader mode

[0ddfell0w]

Lets isFocusable tell the difference between elements that are still focusable despite being invisible

fixes #647

[dylanb]

I would like to see some additional tests:

1. display:none is on an ancestor
2. ancestor is clipped
3. ancestor is off screened
4. visibility:hidden on ancestor

[0ddfell0w]

@dylanb done

[dylanb]

Leaving for @WilcoFiers to determine when to merge

[0ddfell0w]

@dylanb i actually think this might not work, isFocusable() will return True for the unfocusable <button style="visibility:hidden" tabindex="0">Hidden button still not focusable</button>

isFocusable will have the following logic:
is it natively focusable? No, because it's hidden
is it a button: Yes, therefor it's focusable.

[0ddfell0w]

I'll add tests to isFocusable for the disabled/screenreader-invisible cases as part of resolving this issue

[0ddfell0w]

@dylanb @WilcoFiers didn't want to force push over approved changes, so here's another PR with the changes I think are correct: #658

[WilcoFiers]

@0ddfell0w Does that mean we can close this? If so, please go ahead.