Change file extension from .bpmn2 to .bpmn and use content descriptors to associate with correct editor

Currently the BPMN2 editor creates files with extension .bpmn2.

Although the spec does not explicitly indicate what file extension should be used, the examples distributed by the OMG use the .bpmn extension.

The issue is that currently this would conflict with users of the existing BPMN1 Eclipse editor.

Idea from Bob Brodt is to use the same extension, but both editors use the ContentDescriptor approach to determine which editor should be used. See http://community.jboss.org/wiki/EclipseFileContentTypes