This Merge Request creates a function called 'findKnownGerbersLoaded()'. That function in turn
calls the existing 'findNumAltiumGerbersLoaded()' and two new functions:
findNumX2GerbersLoaded()
findNumKiCadGerbersLoaded()
This approach allows a single call to detect Altium/Protel Gerbers, X2 Gerbers, and Gerbers using
KiCad naming conventions.
If it's desired to add detection of Gerbers from other EDA programs, new functions can be written
and then called from 'findKnownGerbersLoaded()'.
Fixes issue https://gitlab.com/kicad/code/kicad/-/issues/4872
Should allow Pcbnew code easier to change and Gerbview code more understandable and easier to maintain.
Code cleaning (remove dead code, add comments).
Minor other enhancements.