This post is deleted!
There is already such a button:
Maybe you need to reinstate it? Try right clicking on the panel bar to see if you can enable it or not...
@pauloaguia Is it in stable or snapshot? Gracias
EDIT: Well, found it. UX of sidebar is not best - I expected it on [ + ] button, found it under right click. Gracias again
Since yesterday, it's in stable
@sukovec The '+' button is for you to add your own panels.
The calendar is a panel built in Vivaldi. Those are always available from the panels context menu.