Make mw.widgets.SearchInputWidget extend OO.ui.SearchInputWidget

Make mw.widgets.SearchInputWidget extend OO.ui.SearchInputWidget

Mixin mw.widgets.TitleWidget instead of extending mw.widgets.TitleInputWidget.

* Remove code that reimplemented pieces of OO.ui.SearchInputWidget.
* Remove code that overrode pieces of mw.widgets.TitleInputWidget.
* Copy the code from mw.widgets.TitleInputWidget that we actually want.

This should result in no functional changes, other than losing the
TitleInputWidget API (some methods and config options) that no one
relied on, as far as I can tell.

Bug: T169194
Change-Id: Ic1482b4c7cfde7d4cf0b8900654bd3a454776010