It's a dropdown select with the ability to add custom options, or a
text field with input suggestions, whichever you prefer.
This is meant to be a replacement for HTMLSelectOrOtherField and
HTMLAutoCompleteSelectField.
In regular HTML mode, it uses HTML5 `<datalist>` element with no
custom JavaScript. This is supported by a wide range of browsers
(IE 10+, modern Firefox and Chrome, Opera 12+).
In OOUI mode, it uses a ComboBoxInputWidget.
Depends on: I14b40884f185fb4e5
Bug: T118119
Change-Id: I954d3d24ed4efe90be9596a1bd9586ba3aee1e23
Move the XmlSelect class to its own file to make it easier to find
and utilize. Helps prevent the use of unnecessary
Html::openElement, Html::element, etc.
Bug: T93234
Change-Id: I66119a2d0eda15569de06c493a0ee302f21deb3f
