FS#11399 by me: fix r26998 for text_viewer

FS#11399 by me: fix r26998 for text_viewer

Restore the old behaviour:

- preferences must be read-write for tv_preferences.c , read-only for
  all other modules -> use pointer to const struct
- init functions must get the plugin buffer + its size as arguments for
  easily adding new functions -> use pointer to buffer pointer and size
  to make allocation easier
- preferences meaning is private to each file and must not be known by
  tv_preferences.c -> move tv_check_header_and_footer() back in
  tv_window.c; also avoid chaining 3 times the callbacks by calling
  tv_set_preferences() only once if more than one preference needs
  changing

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27089 a1c6a512-1295-4272-9138-f99709370657