[dev only] add new standalone mode for startup

[binary1230]

• motivation: I wanted to be able to debug the UI with PyCharm/vscode, but couldn't figure out how to get KiCad+python debugging to work.
• I wanted to set breakpoints to figure out what was going on with downloading and startup errors, which is hard to iterate on when you have to reload Kicad/PCBNew each time
• this is not the most stable thing in the world but gets the job done
• see main.py for instructions on how to launch this

the real work being done here is

• wrap all calls to GetBoard() with a class that can return a fake stub

this helped with creating #242, but is optional.