qquickcontrolsprivate_p.h: add missing includes

Change-Id: I3ace040155a2a095ba23247197cd9484251f2459
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Add ability to dynamically add items to SplitView

Factor out the loop body of init() into a pushItem() function to
make the ability user-accessible.
Can only append items due to the list property limitation.
Takes account of item's fill property

Task-number: QTBUG-35281
Change-Id: Ibb2a5e3cf945bede544cf6bf8eebe13ffd2f79e5
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>