mediawiki.widgets: Create TitleSearchWidget
TitleSearchWidget inherits from SearchWidget instead of TextInputWidget,
showing search results even when blurred.
To avoid duplication, much of the logic from TitleInputWidget has been
moved into the TitleWidget mixin.
Bug: T101169
Change-Id: I335bd912d4f5139646ba05a6a85a0d9ff3d772fa