#701 Allow reading/writing CIF files as XMI files

• Best to review per commit.
• Changes:
  • Allow reading CIF, SeText and ToolDef from XMI files.
  • Use .cifx, .setextx and .tooldefx file extensions for this.
  • Coupled these extensions to the Ecore editor, and gave them a customized icon (darkened versions of the regular file icons).
  • Added a test for reading/writing .cifx files.
  • Also improved/fixed the test_pprint_elim_cdef.tooldef test script.
  • Added documentation for using CIF XMI files.

Closes #701 (closed)