Introduce Kanban boards as a new primitive

[cannibalox]

while waiting for a proper implementation, this css mod can display cildren bullets as columns :

[css] trigger columns/kanban view with tags

a css kanban/columns view for children blocks, triggered by tags usage: tag a block with one of the following tags to display its children blocks in a columns layout : #kanban : automatically adjust the columns width depending on the number of bullets #kanban-small : same as above but with smaller fonts and a hover ‘zoom’ (useful when the classic kanban feels cramped) #kanban-w100 : will set each column width to 100 px #kanban-w200 : will set each column width to 200 px #kanban-w300 : wi…