; LAYER:[layer_num] not inserted if Layer Change G-Code field is changed

[rytz]

If the user changes the Layer Change G-Code to a value of anything other than the commented layer number (which is the default, and is inserted if nothing else is in the field), '; LAYER:[layer_num]' is not inserted, which leads to a failed rendering in LAYER VIEW.

Per @jlewin , '; LAYER:[layer_num]' (no quotes) should be automatically added to the bottom of the Layer Change G-Code if not already present, regardless of what the current value is.