- get_preset_from_id() was defined in file-webp-dialog.c but used in
  file-webp.c only. Move it there.
- Make the preset list available in file-webp-save.h header (since it
  is specifically an encoder attribute) as static, because it is used
  both in the dialog ("preset" choice list) and the main file (for match
  of the "preset" parameter as internal ID to a WebPPreset when run as
  non-interactive).
- Generate the "preset" parameter description from the preset list.
  This way, even if this list were to change (in some hypothetical
  future), the description (and in particular the list of possible
  values and their int match) won't end up wrong.
- "enum WebPPreset" is typedef-ed to "WebPPreset".
- Use G_N_ELEMENTS to compute length of arrays on the stack (equivalent
  to the current code but shorter and simpler to read).
- Many formatting fixes.