Separate 'read_cuetext' functionality from 'parse_cuetext'

'read_cuetext' (renamed to 'webvtt_read_cuetext') now only has one
job: to read in a text until an empty line or EOF (or '-->' sequence)
has been found.

A new internal function called 'webvtt_proc_cuetext' has been
introduced which waits for a successful/finished cue body to be read
from 'read_cuetext', and then finishes the cue in the usual manner.

This allows tests on 'webvtt_read_cuetext' to be more atomic and
separate from the cuetext parsing.