Backport Pasky's changes concerning text_style-related stuff.
It partially includes changes made in following commits:
document/html: struct text_attrib_style -> struct text_style
commit
e1339412068f97be1aa5f91871ba7ebeedbdb5f
document: struct format_attr -> struct text_style_format
commit
070d335796c3e71c5feff8813e681ef4534ee016
document: Unify text style -> screen attribute handling
commit
b66d2bec674f59016ca0708c20249421914b1d2b
document: Move text_style-related stuff to dedicated format.*
commit
db9431465fd77658cfe804947ab336158c1974bd