changing how shell is positioned and screen lip generated

[maave0]

this gets the case closer to the edge of the screen

[maave0]

completely remove vague "shimmy"
Lip thickness will affect how far the case extends above the screen face. Default setting is 0.1mm, barely anything, and 0=flush.
Back thickness is an extra variable now, handled similarly to Joycon back bonus.
There are variables which calculate the shell centerline and the back face, making it easy to align components like the telescoping slider.