Improved user experience for drop down widgets on touch devices

Before this PR, when selecting an item in the popup of a drop down widget (such as a DatePicker, an AutoComplete, a SelectBox or a MultiSelect), the focus goes back to the input element.

On touch devices, focusing an input element triggers the display of the keyboard. This PR prevents the focus from going back to the input element immediately after selecting an element in the popup of a drop down widget on touch devices or when touching the dropdown icon.

The behavior is not changed on non-touch devices (the detection of touch devices is done through `aria.utils.Device.isTouch`).
It is an empty span, there is no CSS rule to make it appear... I don't see the reason why it would take some visible space.
Of course it is always possible to mess up the display by adding some CSS rules to make it appear, but in normal usage, I think it should be fine.
Are you thinking to a specific case for which adding this span would be a problem?

I prefer not to reuse the span of the dropdown icon because it may not always be there (e.g. for the autocomplete without expand button), and it would probably make the code more complex (with a condition to check if it is here or not, and possible impacts on the logic of the DatePicker). I thought it would be simpler to use an independent span for this independent purpose (following the [separation of concerns](http://en.wikipedia.org/wiki/Separation_of_concerns) principle).