Mainly used for mobile actions, and should be appended to the
overlay - this widget assumes it should appear and then fade out
with some confirmation message.
Also moved 'doubleCheck' icon to the ooui definitions, including
an inverted icon that is necessary for this widget.
Bug: T141404
Change-Id: I67a44962eaee6b7bd8cac26dcb5277177fa5d224
