Problem/Motivation
Our current CSS aggregation strategy normally uses 4 different aggregate files (though theoretically, it could use up to 6 separate files.) That's too many files. We can get that down to just 2 files (one for CSS needed on every page and one for CSS needed for a specific set of pages.)
The proposed aggregation strategy is discussed in detail over at http://drupal.org/node/1887922#aggregate
Proposed resolution
This is the first step in a proposed solution outlined at #1921610: [Meta] Architect our CSS. In short, we are changing our 3 aggregate group constants to a single group constant and adding 5 new "weight"-based CSS constants.
This patch is just the first step. It renames the current CSS_* constants to be CSS_AGGREGATE_* and adds 5 new CSS_* constants to be used for weights. It also updates the docs for drupal_add_css().
Remaining tasks
All the other steps outlined at #1921610: [Meta] Architect our CSS.
User interface changes
none
API changes
All the CSS_* constants are being renamed to CSS_AGGREGATE_*.