Made a download worksheet button which allows downloading the worksheet.

Changed the fileuploader to store within the cell id.

Added context menu to _EPROCESS, Pointers.
Changed vad plugin to render pointers rather than ints.

Refactored context menu registration so that pointers can delegate the context menu to the things they are pointing to.

BUG=
R=realbushman@gmail.com

Review URL: https://codereview.appspot.com/142580043