Fix category relationship with markets #37

Updated relationship that Markets have with Categories. Added logic to
market filters to inject categories based on the listings available to a
market. If the listing contains a category, it is then returned as an
"active" category.

Change-Id: Icfb15058ffabd39ff65c8f575e72087fc67c1313
Signed-off-by: Martin Lowe <martin.lowe@eclipse-foundation.org>