Links are lost when exporting to pdf

When we started using cairo it did not support hyperlinks, but now that there is https://www.cairographics.org/manual/cairo-Tags-and-Links.html we should be able to fix the PDF export to keep the links added through "create link" stuff.

Good starting point : https://gitlab.com/inkscape/inkscape/blob/master/src/extension/internal/cairo-renderer.cpp#L516

static void sp_item_invoke_render(SPItem *item, CairoRenderContext *ctx)

migrated from https://bugs.launchpad.net/inkscape/+bug/407701 which contains a link to https://github.com/oxplot/svglinkify as a workaround