add excess walls and distance to TrayInsert so that you can keep sx,sy, yet still perfectly fit inside a box/tray

[ccrome]

Is your feature request related to a problem? Please describe.
I'd like the TrayInsert cells to be of a particular size. However, that size, plus the wall allowances, is not necessarily the perfect size to fit inside my chosen box.

Describe the solution you'd like
I'd like to be able to specify sx, and sy as before, so my cells are just right. I also want to specify an X and a Y, which defines the outer extents of the insert, which corresponds to the inner dimensions of the tray/box that this insert is destined for.

If there is extra space inside the tray that's not occupied by the TrayInsert, the TrayInsert should generate 2 more walls in each required dimension, and extend the extents to perfectly fill the desired X and Y.

I generated the inserts below by adding 2 extra to sx and sy and then manually trimming in lightburn. It would be more convenient to just put it in here.

** Pictures of something similar **

[ccrome]

I guess an additional feature on top of that would be simply give sx and sy as a single measurement, and X and Y, and have it automatically divide up into as many equally spaced cells as possible.

sx = 30, sy = 60, x=354, y = 512 <--- make as many cells as fit
sx = 301, sy = 601, x=354, y = 512 <--- make a single cell in the middle. Silly in this case, but somebody might want such a single-cell box with margin, like maybe for shipping protection.