Fix for issue #46. Correctly use an object map for opaque renderObjects.
Changes order in which asset data is overwritten.

When extending an assetdata or assetmesh, the data linked via
"includes" will override the data of the original entry.
Adapted asset pick filter to support filtering via classNames + test.
First steps towards assets within assets.
Several small fixes - first tests are working.
Several fixes for nested assets + handling of _postTransform assetdata entry.
Added new xflow.get operator used to extract array values by index.
Added match attribute to assetmesh.

The match attribute allows to override multiple assetmesh elements at the same time.
Adapted asset tests.
Started with tests for nested assets.
Finished tests for nested assets.
Merge branch 'instance-improved' into develop
Optimized attribute handler implementations.

- Attribute Handlers are now independent of elements
- Attribute Handlers are constructed only once per element type
- Data is stored in separate "storage" field in ElementHandler
Write element interface into prototype - much memory saved!
Change element implementation to use MutationObserver instead of (deprecated) MutationEvents.

- Added Method XML3D._flushDOMContent that will immediatly flush all stacked DOM changes, updating internal content
- All read-access methods should automatically call XML3D._flushDOMContent to garantuee correct output
Updates several test cases to work with new MutationObserver implementation.