GitLab

* Enable import/export block code as PNGs

This commit adds a feature to download individual blocks as PNG
files. In the PNG file there will be a code chunk that stores the
Blockly XML representation for the block. Dragging and dropping one of
these images into the blocks editor will import that block. This can
be useful for writing tutorials because the images of the blocks will
also contain the code, so one can drag the block image into the
workspace from another page. In order for the cross-site drag to work,
the server serving the document must allow CORS from the App Inventor
server.

Change-Id: I524bbfbef739554884caa31a8b677ce1bcc893d1