support custom shapes other than rectangles

[avxmw]

For some situations it would be nice to have baseplates and/or boxes which aren't rectangles, but L,Z,T-shaped or similar to work around corners of your storage place or not waste space on the grid building a better fitting box around your tool.

https://www.printables.com/model/313164-gridfinitris , https://www.printables.com/model/316113-non-rectangular-gridfinity-bin-parametricish-fusio

I'm not sure what the most user friendly way would be to implement that, maybe an external file with a line by line visual representation like this:

__###_
###_##
#____#

Simply screwing/gluing shapes together might work as an alternative, but then, 1x1 would be all we need to go from there :D