[plt-scheme] recent change to DrScheme in CVS

For details on using the intermediate releases, see

  http://www.plt-scheme.org/anoncvs/

The only recent change to DrScheme is an overhaul of the tabs
mechanism. The current code requires changes to nearly all of the tools
for DrScheme, since much of the state associated with a running program
in DrScheme has moved from the frame to the tab.

 - get-error-ranges and reset-error-ranges are now methods
   of drscheme:rep:text, rather than top-level functions.

 - many drscheme:unit:frame methods have moved onto the tab,
   including all members of drscheme:rep:context<%>

 - fixed bug where cmd-n asks for a file to be opened.

New documentation should be out tomorrow.

Robby