Auto-detect layer-height

[Knifa]

Both PrusaSlicer and Cura add the print settings to the G-code so should be able to extract them out.

For PrusaSlicer it shows up in a big block at the end. e.g.:

; first_layer_height = 0.25
; layer_height = 0.25

This will need things to be shifted around a bit since loading the file only happens when the submit button is hit.