RFC: KLC 3.9

Created by: SchrodingersGat

In this PR I propose a new rule for KLC, 3.9, which checks the values of the default footprint for a component.

1. If the footprint field is empty, then the check passes (ignore any further tests)
2. Footprint field should be set to invisible
3. Footprint field should be of the format "Library:Footprint" (i.e. it must have a single ":" character)
4. Footprint field cannot have any illegal path-name characters e.g. /*?:"<>|

I think such a test (with a good description in the KLC rule file) complements the current set and will also help anyone wanting to submit parts.