Shiny new script which constructs the various icons for the PuTTY

Shiny new script which constructs the various icons for the PuTTY
suite. In a dramatic break with tradition, I'm actually checking in
the resulting icon files as well as the script that generates them,
because the script requires Python and ImageMagick and I don't think
it's reasonable to require that much extra infrastructure on
everyone checking out from Subversion.

The new icons should be _almost_ indistinguishable from the old
ones, at least at the 32x32 resolution. The immediately visible
change is that all the icons now come in 16x16, 32x32 and 48x48
formats, in both 16 colours and monochrome, instead of an ad-hoc
mixture of whichever ones I could be bothered to draw.

The same code can also be adapted to generate icons for the GTK port
(although icons for the running programs don't seem to be supported
by GTK 1 - another reason to upgrade to GTK 2!).

[originally from svn r7063]