Style Engine: Rename WP_Style_Engine_CSS_Rule->set_declarations to add_declarations

[aristath]

What?

Renames the set_declarations method to add_declarations

Why?

set gives the impression that the declarations we're adding will replace the ones that existed. Instead, what the method does is to add declarations to the ones that already exist (and if none exist, then and only then can it be considered set).

[ramonjd]

I'm setting adding my approval 😄