This separates the "newMarker" functions from the DRC class. These
are moved to a new class, DRC_MARKER_FACTORY, which is now responsible for
constructing suitable markers from BOARD_ITEMS.
The reasons for this are:
1) Allow DRC related functions to access theese functions without having
to bake these functions into the main DRC class.
2) Thereby simplify the DRC class interface
3) As DRC_MARKER is now a small class with public interfaces, it's
possible to put unit tests on these functions.
The motivation here is to allow to start to split out pieces of DRC
into separate classes.
John Beard