PHP DropdownInputWidget: Restore $overlay=true default
PHP DropdownInputWidget: Restore $overlay=true default

We should probably make this the default in JS
too, but that would be a breaking change.

Partially reverts I641e389c8823.

Change-Id: I4ac402a74c0a675414c8cb4ada0c24391d638a21

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