Make text selection handles behave in a Flickable

Prevent stealing but not before the drag threshold has been passed.
This makes it possible to flick over the handles, but the press won't
get easily stolen by a Flickable after moving the handles. The same
technique is used in Slider.

Change-Id: I8eac1bf84ada8bb2334f5ed3985f14a520ecc3c5
Reviewed-by: Liang Qi <liang.qi@digia.com>