sometimes after GetCandidate dbus call, zero-input-fetch-size and
zero-input-candidates is not updated. This was because the async
function callback didn't update the metadata in some places.

- update initial fetch size and fetch size to be 1+ candidates needed for
  target page.
- removed zero-input-build-candidates-complete function,
  each async call should have its own callback function, don't reuse this one.
  in callback function, post GetCandidate metadata should be updated.

- dropped Package-Version metadata.
- zero-input-full-width-mode renamed to zero-input-full-width-p.
- zero-input-toggle is now an obsolete alias for zero-input-mode.
- use `defvar-local' when variable is always buffer local.
- moved test data to debug/test files.
- fixed typo. zero-input-pinin-service > `zero-input-pinyin-service'.

since this is a backward incompatible change, I set version to v2.0.0.
all symbols and functions is now prefixed with zero-input.

This change is suggested by melpa reviewer.