Asset database #1604

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.