their type, by using the more abstract PLUGIN::Footprint*() functions.

This is an intermediate phase only, other changes will be necessary as library table support comes in.
Encapsulate usage of library path searching, since that will go away as library table support comes in.
Add FOOTPRINT_EDIT_FRAME::{get,set}LibPath() and FOOTPRINT_EDIT_FRAME::{get,set}LibNickName() functions
to provide this encapsulation.