Add clickable link to STK website in About dialog

[scootergrisen]

I suggest adding a clickable link for the STK website in the About menu.

As with a lot of other software projects they often have such a link in the About dialog so it's easy to visit the website.

At the moment it says "Visit us at supertuxkart.net" for about 4 seconds when you visit the About menu. But it's not clickable.

Also when starting STK there is sometimes this tip. Also here the URL is not clickable:

You can visit https://supertuxkart.net/ for more information about the game.