The automatic completion of headers in the MANIFEST.MF does not include
Eclipse-SourceReferences, which can be used to add meta-information to
the headers to permit the bundle to be resolved from a source code
repository as per:

  https://wiki.eclipse.org/PDE/UI/SourceReferences

Allow this to be suggested in auto-completions.

Bug: 469803
Signed-off-by: Alex Blewitt <alex.blewitt@gmail.com>
Change-Id: Ibaf27ff6ffdd3c35053c50f1aaba1926cddb540d