api: add waitForElementState('disabled')

[dgozman]

Allows waiting for the element to be disabled.

References #2353.

[JoelEinbinder]

This looks fine, but it made me read all of the _isElementDisabled code we already have. It looks very strange to me. We accept role=button, but then don't look at aria-disabled? And we look at <button>/<input>/<select> but not <textarea>. Additionally these elements could be disabled without the attribute set if they are in a <fieldset>. <option>/<optGroup> might be an edge case, but they can also be disabled.

[dgozman]

This looks fine, but it made me read all of the _isElementDisabled code we already have. It looks very strange to me. We accept role=button, but then don't look at aria-disabled? And we look at <button>/<input>/<select> but not <textarea>. Additionally these elements could be disabled without the attribute set if they are in a <fieldset>. <option>/<optGroup> might be an edge case, but they can also be disabled.

Sounds like we can improve stuff by a lot! Would you like to look into that?