Round title buttons

Make button.titlebutton:not(.appmenu) round

• use even sides for the titlebuttons so the result is a circle and not an ellipse
• add maximum border radius
• set min width/height to 0
• adjust margin and padding to match the previous look
• exclude appmenu

:hover

:active

Screencast_from_08-01-2019_10_27_13_AM

As discussed with @jimmac @bertob and @aday