Doxygen: fix proprietary browser references

Doxygen generates API reference documentation from special comments extracted from source files.

It supports generation of graphs using 'dot', part of GraphViz. These graphs can be generated as SVG files, but not all browsers suport SVG. So if SVG isn't supported, a list of suggested browsers is displayed to the user browsing the generated webpages. By default the list contains proprietary browsers.

This patch changes that list to suggest only freedom respecting browsers (which support SVG rendering).

I also filed this issue to the gnu-linux-libre mailing list, but got no feedback so far. I decided not to wait and fix this issue here in Trisquel.