Added simple proof-of-concept support for running PSL scripts from

Added simple proof-of-concept support for running PSL scripts from
input bindings.  They will work only with the latest CVS; otherwise,
./configure will disable them.  There is a new command, 'script',
which takes a single argument, also called 'script', containing PSL
code (currently PSL requires a main() function).

Erik Hofman has written some more elaborate code for triggering PSL
code from drop-down menus and scheduling events; I will look at
integrating that next.