TextInputWidget: Use MutationObserver for #onElementAttach support
TextInputWidget: Use MutationObserver for #onElementAttach support

This makes it work in Firefox and shouldn't affect other browsers
negatively. See T89399#1084383 for the story behind this change.

Tested with Opera 28 (like Chrome), latest Firefox, IE 11, Opera 12.

We should get rid of this long-term because it really sucks.

Bug: T64084
Bug: T64174
Bug: T89399
Change-Id: I681038ea42740f3275733246813eb8d86176c408

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui