Use stylesheet_import to include CSS

Looks good to me. I think this is ready to be committed (but I'll let the module author make that status change, of course). I've produced a patch which does the same for 4.7 (posted in a separate issue).

drupal_set_html_head() shouldn't being used for inclusion of stylesheet. For that purpose drupal_add_style() was introduced.

See attached patch, and yup.. It's ready to rock and roll!

no no no no no

First of all, this is a duplicate of bug 53072. (I suppose since this one is older, technically 53072 is the duplicate.)

Secondly, theme_add_style should NEVER NEVER NEVER be used for including a module-specific stylesheet. Doing so prevents properly-written themes from overriding the module CSS if they so choose. Please see my (exceptionally wordy) explanation from a core issue.

Perhaps there should be a handbook page about this or something? or maybe I should do a documentation patch?

To clarify:

* Yes, we should use theme('stylesheet_import') -- and a patch has landed which does this.
* No, we should not use theme_add_style... and no module ever should unless it's trying to OVERRIDE the theme for some reason (i.e. CSS that was manually entered or created by the end user)