update gitignore

[lifeisafractal]

This PR has some suggestions for updating .gitignore to match what I've seen as best practices for working with KiCad and FreeCAD in git. In addition to updating .gitignore, existing committed files that would now be ignored were removed with the following shell command:

git ls-files -ic --exclude-standard | xargs  git rm

.gitignore Additions

• Ignore *-backupds/ folders
  • KiCad will put periodic backups of the project in a folder named [PROJECTNAME]-backups/[TIME].zip. This is great for local use, but can clog up git with unneeded info. It makes sense to let git be the source of history/check points in the repo.
• Ignore *.kicad_prl
  • These exists once per-project and hold local settings like which layers are visible and the such. There is no design information in this file, and KiCad Docs call out that is doesn't need to be version controled. This file pretty much always gets updated when you open a project, so ignoring it makes it easier to poke around projects without dirtying the working tree.
• Ignore *.FCBak files
  • These are FreeCAD backup files. Same story as KiCad backups described above.