Make Text's rawSrc setter work like Asset's.
Make Text's rawSrc setter work like Asset's.

Previously the Text one removed the asset from the graph and re-added it afterwards, which had the unfortunate side effect of killing its incoming relations.

This fixes a processImages problem in assetgraph-builder where a regression test is about to be added.

papandreou/assetgraph,papandreou/assetgraph,papandreou/assetgraph,jscinoz/assetgraph,jscinoz/assetgraph,jscinoz/assetgraph,assetgraph/assetgraph