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.
Doc: remove dead link to gtk_widget_override_text()

The function doesn't exist.
gtktexttypes: remove inline_byte_begins_utf8_char()

The function was used only in gtk_text_byte_begins_utf8_char().
gtktextattributes: remove outdated comment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.
gtktextattributes: include the right headers

Some function prototypes in gtktexttagprivate.h are implemented in
gtktextattributes.c.
GtkTextView: remove dead code
GtkTextView: minor code style fixes
GtkTextTagTable: simplify a bit the code