Auto-include theme CSS files with theme override

The attached patch adds two simple and related bits of automation.

1) If a module is active, and there is a file in the module's directory named modulename.css, it will automatically be included in the page head.

2) However if modulename.css exists in the currently in-use theme, then that file will be included instead, and the one in the module's directory will be ignored.

Both of the above happen automatically with no code needed on the part of a module or theme.

Here's why:

Currently, if a module wants to include a CSS file, it has to do so manually by calling theme_add_style(). I keep seeing contrib modules put that call inside hook_menu(), which makes not the slightest bit of sense. :-) The exception is core modules, which have all of their CSS rules dumped into the oft-malaigned drupal.css file. Those rules are then included always, even if the module to which they correspond is not active.

Moreover, if a theme wants to style something differently from the way drupal.css or the module's own CSS file define it, then the theme author has to specifically override every part of the rule in question. We've had the discussion about how annoying that is many times, so I will not repeat it here. :-)

With this patch, Drupal will pick up module CSS files automatically if they exist. The module author need not pollute the already-abused hook_menu() with it. Moreover, theme authors can trivially override a given module's CSS file by simply copying and pasting the file and modifying as needed, or even making it an empty file to have no rules at all.

That also paves the way for the death of drupal.css, now that core modules are in their own directories (yay!). With this patch, drupal.css could be split up into modulename.css files for the appropriate core module and placed appropriately. Then, with no code changes, all of the extra stuff in drupal.css that you never use if a given module is not active goes away, and overriding drupal.css in themes becomes trivially easy; copy/paste/modify the file in question and you're done.

Known issues:

- I'm not certain that I have this code in the best function. :-) I've put the logic into theme_get_styles(), but I can see an argument for it in drupal_get_html_head() as well. I defer to the big people on this one.

- Currently it does not handle different media types in the slightest. Not all contrib modules do either, to be frank, but I am open to suggestions on how to handle those.

- It may be faster to cache this information rather than rebuilding it every time. Again, I defer to those with more experience with caching.

Feedback requested!