Box fixes

This branch started out just to fix #604, but ended up fixing various other things out of necessity along the way. First off it fixes problems with cursor up/down and tab key handling in the NodeGraph - without that I couldn't get inside a Box. Then it turned out that many of the menu items had the same problem as the Box promotion ones, so I fixed those too. Then it turned out that even in a read only node ui, the plug labels were writable, so I fixed that too.