[text] Text loses its textString when not in DOM

[fbornhofen]

Check branch text_serialization_fix for test case.

A Text's textString only seems to be persisted when the Text is part of the scenegraph.
This breaks LosProfesores in Webwerkstatt.

Will try caching of text chunks.

[fbornhofen]

Update: textChunks will be persisted and their storedString seems correct, too. Will look into restore.

[fbornhofen]

Update: storedString is present in deserialized TextChunk, but textString is empty. Need to update accordingly.

[fbornhofen]

Now not deleting TextChunk's storedString on restoring. Not sure if this is final.