Save stage/pentrails as costume (for animations)

Created a new block: "save screenshot as costume named %s"
%s takes the Costume name
Block is created under sensing
Multiple screenshots with the same name have a counter associated with them so that names are not repeated. 
Users can go to the costumes tab and right click on them to export them as png.
All the changes are in objects.js
Ugh! A global variable...