Tweak the icon script, and the generated icons, to more closely

Tweak the icon script, and the generated icons, to more closely
match the original icons. (Apparently I managed to introduce errors
while transcribing the originals for detailed analysis.)

While I'm at it, add the obviously useful `make install' target in
icons/Makefile, and fix the svn:ignore property on the icons
directory.

[originally from svn r7068]