Rework header with custom title/color + move about info

Related to daiquiri!791 (merged) daiquiri!794 (merged)

• Use a blend color with opacity to allow to use any color for the background.
• Propagate header_color and header_title to the header color.
• Replace "Daiquiri UI:" by a clickable icon dropping the about box
• The about box was removed from the user drop menu