Introduce small common position class in scanning and parsing

Currently the setext scanner and parser code uses an EMF-based position class for storing file positions. The consequence of that decision is that all applications that parse an input file using a setext generated scanner and parser must import EMF.

To make such applications easier to port to a different environment, a new common position class is introduced, thus moving the decision for using the EMF-based position class to the AST tree building code instead.

Addresses #307 #43 (closed)