The rough begining of the buffer-combine mode.  This will show parts of
multiple files in one buffer.
It now has a marginally functional display buffer
Beginning to make the display buffer write back to the source buffer if edited
A mostly functional buffer-combine mode
Show CSS now uses a display buffer to show elements from any number
of other files combined into one buffer.  If the display buffer is
edited, the edits are sent back to the source buffer.
Small tweak
For each tag, the id and all the css classes from all source files are
displayed together in the display buffer.  At the moment, it is to slow
to be used.
run-with-idle-timer is now used, instead of keypresses to trigger
the code.