allow for editing during indentation

When a file is being block-indented, allow the user to
modify other open files. There's no reason why other
open buffers need to be locked.

Story:

The user has two files open. He selects all and hits
tab. The indentation takes a long time. While it's
going, the user switches to the other open file and
edits it. When indentation is completed, some
notification is given to the user (preferably not a
dialog box. Maybe the color of the file's name could
change, or notification could be given on the status
bar).

-- Eric