TextInputWidget: Reduce CSS output by enhancing unselectable behaviour
TextInputWidget: Reduce CSS output by enhancing unselectable behaviour

Removing `.oo-ui-unselectable()` mixin from icons/indicators, where it
doesn't provide any effect and reducing selector output.

Change-Id: I2bf70e98e8883e9a8d1ec9e261914249038ac36b

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