Asset database

*Make Image nodes store their bitmap data in an asset database stored on the document*

Right now, if you drag an image into the document, the Image node itself stores the data (as part of a value input which we hide, but would otherwise appear in the Properties panel just like any other parameter like a number or a color). We want to instead have each document store its blob data (such as images) in an asset database, and then it would be accessed by URL as `graphite://d0ce5747-6916-42a0-88d9-a623ac5d927f` for example.