Support image and sound literals or storage

[neilccbrown]

One pending issue with having a graphics world in Strype is that we need to be able to load images and sounds somehow. For cloud-stored projects the obvious possibility is to allow loading separate image and sound files from the same folder in the cloud store. However, for local stored projects this is not viable due to browser security restrictions. I suggest two possibilities:

• To be able to store images, sounds (and similar, like CSVs?) in a dedicated "files" section of a project, to which you upload once and thereafter it is stored in the project file.
• To be able to put images and sounds into the code as literals, like I think DrRacket does.

Both options require storing the file data in the project, which runs the risk of overflowing the allowed browser storage, and increasing file size when downloading/uploading projects.