Improve "about"-dialog

[ibogicevic]

• When clicking on a link in the about-dialog, the color of the link changes (because the page has been visited) and a rectangle occurs around that link. The color should not change and the rectangle should not occur.
• The text "GPL General Public Licence v3.0" should also be a link to https://www.gnu.org/licenses/gpl-3.0.en.html
• There should be an additional empty line after this gpl link
• The list of contributors should get nicer bullets (not only "-")
• The list of contributors should be added by an item "...and many others"

[ibogicevic]

Moving this issue to milestone 4 as there is no progress.