Add keyed properties for issue occurrences in footer lines

For for example a footer like:

  Related-To-Bug: bug#42
  Fixes-Bug: bug#4711

would now also add occurrences "footer-Related-To-Bug" for issue 42,
and "footer-Fixes-Bug" for issue 4711. Thereby, we allow to formulate
rules that fire depending on where an issue has been mentioned in the
footer.

Change-Id: I37a0b3bcd555d3cc7a34c0143e6229a0d86ad540