The `prev()` function doesn't take nodes other than elements into account. Also we could simplify things a lot using the `previousElementSibling` property. This property isn't fully supported in IE, it only works for elements, but since the `element` variable is an element, we can safely use it here.

I've also added an additional test.

I don't think we had this issue in v4, since we relied on jQuery back then.

Ref. https://developer.mozilla.org/en-US/docs/Web/API/NonDocumentTypeChildNode/nextElementSibling

* Close modal with keyboard=true & backdrop=static

* Enable and fix all occurrences with no-mixed-operators rule
* Take care of the max-depth warning in button.js

Signed-off-by: mhatvan <markus_hatvan@aon.at>