GtkTextView: minor code style fixes
gtktextattributes: remove outdated comment

GTK_WRAPMODE_CHAR etc don't exist anymore, it seems that the names have
changed. Anyway the comment is not really useful.
Doc: move the GtkTextAttributes struct

It's more logical to have the GtkTextAttributes and GtkTextAppearance
structs together. And it creates a separation between gtk_text_tag and
gtk_text_attributes functions.
gtktextattributes: include the right headers

Some function prototypes in gtktexttagprivate.h are implemented in
gtktextattributes.c.