Optimize listmodel and allow nested elements from worker script.

Optimize listmodel and allow nested elements from worker script.

Added support for nested listmodels when used from a worker script
thread. Optimized the implementation of ListModel, especially the
performance of appending a large number of items. Added a batch
append mode (with an array of JS objects) to reduce the overhead
of calling from JS into native code for each append operation.

Task-number:QTBUG-21508
Change-Id: I07b381dc3e8200d92d6e0af458df8850d78b510f
Reviewed-by: Martin Jones <martin.jones@nokia.com>