List all changes related to a build.

 - to do this, the column 'parent_changeid' was added to the table changes

Note:
 - I added a tab 'changes' for the build webpage. I think this should be modified in order to have something more readable (but my angularJS skills are very limited).
This comment applies a number of other places in the patch, too, that I won't comment individually.  One technique is to put the db method into a short variable, e.g., `gssfb = self.master.db.sourcestamps.getSourceStampsForBuild`.