Adds `stackCandidateTouchResponder` to SC.Touch.

Allows views to add themselves as a fallback responder (e.g. during the `captureTouch` scan) without having touchStart and touchCancelled needlessly called on them if they end up not receiving the touch.
Fixes a bug where native touch handling was impossible in ScrollView.
Strengthens behavior of SC.ScrollView#delaysContentTouches: it's now the responsibility of any descendent view which handles the touch to pass it back to ScrollView with SC.Touch#restoreLastTouchResponder. Includes tests for new behavior and improves coverage for existing behavior.