This adds the "gui log" command, which pops up a window you can use
for logging.
There are also new "gui" subcommands for sending output to a log
window: gui print, gui output, gui printf, and gui dprintf. These
all work pretty much like their non-gui counterparts, except that the
output goes to the log window.
If you have more than one log window you can specify which one to use
with an "@", like gui print @1 value
This adds the "info windows" and "delete window" commands.
"info windows" unfortunately overlaps with the TUI's "info win".
So maybe we should rename this one.
This also adds a new "toplevel" component that knows how to track
windows.
