Add transparent background option for artboards on option bar

[rikkarlo]

Hello,
recently I struggled trying to find how to turn the artboard background to transparent, I found it in the preferences very hidden.
For a more userfriendly experience it would be nice to have the option directly available on the option bar when selecting the artboard or maybe in the export window in the dropdown menu where you select the type of export (together with selection area, only selected, artboard, all document...)