#2051 - Revise how Gallery_View returns combined JS and CSS.

Changed Gallery_View's JS and CSS combining to:
- have get_combined() combine all groups if called without arguments.
- ensure the "core" group is combined first.
- always put links where get_combined() is called, even if combining is disabled (otherwise the order isn't preserved).
- add key as cache buster when combining is disabled.
- make "LOOKING FOR YOUR CSS/JAVASCRIPT..." comments in get_combined(), not in the theme itself.
Also, revised xss_data.txt golden file (line number changes only).