Introduce Util::FileAssoc enum

The string comparison currently used to pass around file association
info is unscalable in a growing project.  To correct this, the enum
FileAssoc was added in the Util namespace (in file Util.h).

Additionally, several functions were added to Util.h/cpp to allow
file association to be checked before the creation of a CodeFile object.
This "pre-check" is important when creating CodeFile child objects
polymorphically, to determine which child-type to use.

This commit does not effect CodeFile methods yet, it only
introduces the enum to be used soon.