mw.widgets.UsersMultiselectWidget: Compat with mw.special.preferences.confirmClose

mw.widgets.UsersMultiselectWidget: Compat with mw.special.preferences.confirmClose

Replace `<input type="hidden">` with a `<textarea>` hidden using
`display: none`, because the former does not have separate 'value'
and 'defaultValue' properties, and the Special:Preferences script to
enable/disable the "Save" button depends on them to work correctly.
We use a similar approach in OOUI (RadioSelectInputWidget).

Also correct a code comment (we need to trigger a 'change' event for
any kind of input, not just `<input type="hidden">`).

Bug: T192147
Change-Id: I2351bc752f9cf25a607f4b197c132062d52ecb1e