PEX96-Alt

[P3X's frame](https://www.thingiverse.com/thing:2188747) is finely designed, but transferring my Lizard95's electronics to it made the whole model a tad too heavier of the original than I thought.

This is my attempt at slimming the build by removing some material. While I was at it, I also redesigned the motor holes to make them align better for my 1104 motors: with the "Plus" orientation of the original frame, the cables came out from the motor not on the arm's support but sideways. With my "X" orientation, the holes now grant that the motors' cables run straight on the arms up to the central stack.

My remix is based on the _Top v0.3_ & _Bottom v0.5_ files.

The expected weight loss is 2-3 grams.

#### Summary of changes

+ Moved motor holes from "Plus"-style to "X"-style.
+ Removed central ribs from front, back, left, right axes.
+ Shortened width of the front bracket between supports.
+ Shortened motor cables' support ribs on arms.
+ Shortened frame protrusion into a