[reference] Use xdg-open instead of firefox

[iacore]

When I used ide7, I noticed it's not showing anything. I tried to fix opening browser on Linux, but I don't know Seed7 enough.

On Linux, opening browser should be like this:

• use xdg-open http://example.com/ (free desktop)
• run $BROWSER http://example.com/ if env var BROWSER exist
• hardcoded browser paths

Also, I noticed this function will not fail if non of the browser exist (browser.program is empty).