Defer GUI thread calls in the Lottie animation builder when needed

When the item generator is running in async mode, we need to defer
certain calls until after the worker thread finishes and the item tree
is returned to the main thread. This uses the API introduced for this,
which defers the call if we are in async mode and calls it directly if
not.

Task-number: QTBUG-147710
Change-Id: Ia6e9d5d6a6a762f0ae9066a8684cd224f7bbfcfc
Reviewed-by: Kwanghyo Park <kwanghyo.park@qt.io>