text_viewer: cleanup & bugfix

text_viewer: cleanup & bugfix

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