Update MouseEmulation class in order to allow scrolling in lists without additionally selecting items 

Hello,

we've a qx desktop application that has to run on mobile devices too, after enabling the mouse-event emulation most things work fine, except for lists. 

When trying to scroll a list using a mobile device (e.g. Ipad), the list scrolls but also selects the item beneath.

The idea behind this pull request is to delay the "mousedown" event for a given period of time (80ms felt best) or until the "touchup" event was received.

I do not know if this messes up the event handling, due to the fact that a preventDefault is now asynchron.

regards 

Fabian