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

apps/plugins/text_viewer/tv_pager.c

@@ -85,7 +85,7 @@ static void tv_change_preferences(const struct tv_preferences *oldp)
     tv_seek(0, SEEK_SET);
 }
 
-bool tv_init_pager(void)
+bool tv_init_pager(unsigned char **buf, size_t *size)
 {
     tv_set_screen_pos(&cur_pos);
     tv_add_preferences_change_listner(tv_change_preferences);
@@ -95,7 +95,7 @@ bool tv_init_pager(void)
 
     line_pos[0] = 0;
 
-    return tv_init_reader();
+    return tv_init_reader(buf, size);
 }
 
 void tv_finalize_pager(void)