[O] Folding Source Blocks

[Ian Barton]

If I have a large begin_src block it can make reading the surrounding 
text difficult, as the reader has to keep paging up and down. Ideally it 
would be possible to fold src blocks, but I can see that would be 
inconsistent with org's structure. I could put the block in a draw, but 
was interested in what others do.

Ian.