add a tab and slot export option

[PlantLily]

as an alternative to gluing; having tabs proportional to the size of the edges they connect (with a limit to size and option to double up if an edge is too big) would be useful.

by having a centered hexagon tangent to the edge, and the corresponding edge to have a slot as wide as the widest part of the hexagon it is possible to have a design that doesn't require any glue.

here's a mock-up i made in inkscape using the export of the default cube: