[plt-scheme] 204.10

The exp-tagged code in CVS for MzScheme and MrEd is now version 204.10.

The changes are minor:

 * Changed WXME clipboard format to include the format version, so cut
   and paste will work correctly from old to new versions of DrScheme.

 * Added `write-editor-version' and `read-editor-version', which
   provide the version-tagging support for other tasks.

 * Fixed a bug in blocking output when the "device" write_string can
   sometimes perform a partial write, even in blocking mode. (The only
   such device was the SLL driver, which suggests that the bug started
   as a specification error. Still, there's no point forcing the loop
   into the device.)

Matthew