Need copy, undo and redo support about blockly HOT 3 CLOSED

I just saw this is in the Future plan wiki.  Glad to see but I would recommend 
that this issue along with save/load are critical for usability.  Additionally, 
a user should not loose a page of work by clicking on the browsers back button. 
 I will add this as a comment on the save issue since I did not see it 
mentioned anywhere.

I think its worth to isolate each function to different issues

1. save/load, as issue 7
2. copy/paste
3. undo/redo
4. functions with keyboard shortcut (ctrl-c,  ctrl-x, ctrl-v, ctrl-z, ctrl-y)

Closer analysis of the cut/copy/paste triad show that they is not wholly 
appropriate to Blockly.  Think about when you use cut.  That operation is only 
used when one is moving something.  In Blockly one would never cut a block and 
paste it somewhere else, one would just drag it.  As a result the three 
clipboard operations boil down to a single operation: Duplicate.

Admittedly, advanced applications of Blockly might have a use for 
cut/copy/paste if one were moving content between windows.  However this is 
very tricky since the windows might not know about each other's block types.  
E.g. the Maze demo has a different set of blocks when compared with the Code 
demo.

Since 'duplicate' is the overwhelming use-case when people think of 
cut/copy/paste in the context of Blockly, I've added this function to the top 
of the context menu.  Let's wait until we see multi-window Blockly applications 
before implementing the other three.

As gasolin says, undo/redo are a very separate set of operations.  We have a 
plan for these which involves something rather special.  But we're being 
temporarily blocked by a Google-internal issue.  It will remain on the Future 
list for right now.

You also mentioned clicking back and losing work.  This was added some time ago 
as a result of feedback.  Blockly's demos now check that there are blocks 
present, and if so issue a warning dialog.

Now that 'Duplicate' works I'm going to close this issue, with the 
acknowledgement that undo and redo are very important and will be implemented 
as soon as possible.