Instead of relying on SVG rendering (and rsvg pixbuf loader), we can ship
pre-generated symbolic icons. They are faster to initially render and help
avoid some memory fragmentation caused by rsvg parsing a bunch of XML at
startup.

This also moves some of the designer files off into src/. The render
scripts never really worked well for me, but I anticipate they'll need
some fixups the next time we edit iconography.