Add Top and Bottom override on footprints

[MajorArkwolf]

What is the problem?

Some footprints can be "double-sided" but in reality once broken into individual parts may need to be strictly on one. An example of a footprint that is used for the top, but a component on the bottom is Kaihl V2 Hotswap key where the bus bar is mounted through the bottom and faces the top.

Currently, there is no way to produce a placement file that accounts for, therefor manual editing of the positions file needs to be done to correct the side.

How does this PR fix this?

By implementing a layer override function, similar to what is already done for rotation and position, we can override the kicad footprint and instead have the positions file reflect the expected output that JLCPCB is expecting.

To apply an override open the schematic, then click on a symbol and add the following field 'JLCPCB Layer Override' followed by either top or bottom.

What testing has been done?

• Tested without any changes, footprint was on the wrong side
• Tested after changes with valid values (expect the layer to swap to either top or bottom)
  • [x] t
  • [x] T
  • [x] top
  • [x] TOP
  • [x] b
  • [x] B
  • [x] bottom
  • [x] BOTTOM
• Tested with a few invalid values (expect no changes to the default layer)
  • [x] i
  • [x] `
• Validated that output on JLC website reflected said change
• Validate that offsets are still correctly being applied after the layer swap.

Before

After (Not offset)

After (Post offset)

[MajorArkwolf]

@bennymeg I ran into the bug with the other implementation causing the plugin to crash. I then tried to implement it on main but was getting weird bomRowLimit error. So I ended up branching of the last release and implemented my own solution.

I have tested it and it appears to be working well :)

[bennymeg]

Closes #64, #69.

[bennymeg]

@MajorArkwolf What was the error you got?