Improve usability of Constants Editor
A demo of this dynamic editing can be seen at http://webempoweredchurch.com/fileadmin/proj/misc/constants_editor.mp4
In addition, the HTML output of the submodule can be cleaned up to produce more semantically valid HTML.
There's still plenty of room for discussion on this patch. Is a trash can the appropriate icon or should we have something that indicates close or rollback instead of delete.
We'll also need intregration with the new docheader for the Templates module once that is complete.
As such, this is an attempt to get the patch out there for inclusion in 4.2, but I expect there will be some refinements needed.
(issue imported from #M7336)