ButtonGroupWidget: Change `cursor` on `.oo-ui-buttonElement-active`
ButtonGroupWidget: Change `cursor` on `.oo-ui-buttonElement-active`

Bringing `cursor` behaviour in alignment with ButtonSelect/OptionWidget
as introduced in Ia8c782c99621b39e48d39d9a389bd024ae231062.

Change-Id: Idc2ef8d98312b7182b903215986596f13a696411

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