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/towers.html

@@ -0,0 +1,19 @@
+Towers
+<p>
+Fill in the grid with towers whose heights range from 1 to the grid
+size, so that every possible height appears exactly once in each row
+and column, and so that each clue around the edge counts the number
+of towers that are visible when looking into the grid from that
+direction. (Taller towers hide shorter ones behind them. So the
+sequence 2,1,4,3,5 would match a clue of 3 on the left, because the
+1 is hidden behind the 2 and the 3 is hidden behind the 4. On the
+right, it would match a clue of 1 because the 5 hides everything
+else.)
+<p>
+To place a tower, click in a square to select it, then type the
+desired height on the keyboard. To erase a tower, click to select a
+square and then press Backspace.
+<p>
+Right-click in a square and then type a number to add or remove the
+number as a pencil mark, indicating tower heights that you think
+<em>might</em> go in that square.