Fixes bug that prevented native touch handling in a ScrollView.

Strengthens `ScrollView#delaysContentTouches: NO` to go entirely hands-off with touch events. This allows the triggering of native touch handling; it also makes touch handling more stringent but more predictable: it's up to content views to pass control of the touch back to the scroll view (via the new `SC.Touch#restoreLastTouchResponder` method).

This basically turns delaysContentTouches into a "hand-holding mode" toggle. The default remains YES, which means that default mode remains easy mode; doing useful and interesting things in advanced mode meanwhile becomes much simpler.

Adds the concept of a candidate touch responder, which allows views like ScrollView to suggest themselves as fallbacks without having to bother handling unnecessary touchStart etc. events. Very handy.