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

cleanup:
    - don't use enum in struct / return values
    - don't use a getter for preferences but a global pointer
    - explicitely make enums start at 0
    - use static tables for header/footer settings
    - remove unneeded memset before strlcpy
    - use static buffer allocation, not dynamic
    - check header/footer preferences before using the callbacks
    - don't include font filename in archos player preferences (break
      file format)

bugfix:
    - statically allocate old preferences in tv_set_preferences()

Sometimes I can read a file on Clipv2, but it still aborts quite often
refs: FS#11399

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@26998 a1c6a512-1295-4272-9138-f99709370657

new text viewer plugin: text_viewer.rock.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@26571 a1c6a512-1295-4272-9138-f99709370657