Rather than increasing the size of my switch statements yet further, have

Rather than increasing the size of my switch statements yet further, have
a bunch of function pointers associated with each window to do things like
updates and click handling.  This is all looking disturbingly object-oriented.
.
While I'm here, separate out the about box into its own file, shared by PuTTY
and PuTTYgen.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2850 cda61777-01e9-0310-a592-d414129be87e