Introduce a mechanism in this source tree for building the container

web pages for the Java applets. Previously, those have all been
maintained by hand in my website's svn area, which is a bit silly. Now
we have a file per puzzle in the 'html' subdirectory which contains
the puzzle's name, one or two attributes, and the instructions snippet
to go below the puzzle applet; and then there's a Perl script that
builds all the real web pages out of that by adding in the parts
common across all files: the header, footer, and middle fragment with
the <applet> tag and resizing bits and pieces.

One piece _not_ checked in here is the footer text specific to my
hosting at chiark, which I think does still belong in the www area. So
Buildscr doesn't actually build the web pages; it just delivers the
bits and pieces by which my nightly snapshot script will be able to
run the program that _does_ build them, passing that footer as an
extra argument.

[originally from svn r9780]

html/blackbox.html

@@ -0,0 +1,16 @@
+Black Box
+<p>
+Determine where the hidden balls are in the box, by observing the
+behaviour of light beams fired into the box from the sides.
+<p>
+Click in a square around the edge of the box to send a beam into the
+box. Possible results are 'H' (the beam hit a ball dead-on and
+stopped), 'R' (the beam was either reflected back the way it came or
+there was a ball just to one side of its entry point) or a number
+appearing in two squares (indicating that the beam entered one of
+those squares and emerged from the other).
+<p>
+Click in the middle of the box to place your guessed ball positions.
+When you have placed enough, a green button will appear in the top
+left; click that to indicate that you think you have the answer.
+You can also right-click to mark squares as definitely known.