Say hello to QQuickItem::grabToImage().

[ChangeLog][QtQuick][Item] Added functions QQuickItem::grabToImage()
and Item::grabToImage() to allow grabbing of items into
system-memory images.

Change-Id: I76cd73bb62f7440569c6fce63d63528559845721
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Opt-in that QSGSimpleTextureNode can own the texture.

Having to subclass just to delete the texture is a bit
tedious.

Change-Id: I26c7e0b5c053ba70e004949ead748e50756d9b39
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>