Gradle makes pngs: it builds

This branch can generate png icons from svg files at build time. It also introduces new, precolored SVG templates. I found that while the cleaning is not elegant and proper incremental building is missing, I currently don't know how to make it substantially better. Please test with some of your icons before merging. Ideally also follow 'Brief introduction' from https://gitlab.com/xphnx/twelf_cm12_theme/wikis/icons-en to check whether it's correct and sane.

For other details, let me quote build.gradle:

// This scripts builds the icon theme, rendering png files in build-time.
// Should be portable, doesn't require rsvg-tools or inkscape at all.

// Somebody who knows groovy/gradle should have a look at that
// and correct all the dirty hacks I've used here, willingly or unwillingly.

// For some reasons the former template was so messy, that the icons
// based on it do not render. Please paste the icon onto the new template
// (svg-icons/twelf-icon-template.png).

// It also generates pre-colored svg icon templates
// in a directory named 'templates'.