synthesize.h now contains the definitions STRESS_IS_... that should be used with code related to syllable stress.

Note that isBreak and other defines were renumbered so that stress definitions could have values 0-6.

Possible TODOs:
1. Unify with terms used with phonemes, i.e. keywords like isDiminished in compiledata.c and stress_type in phsource/phonemes
2. Add functionality and documentation about STRESS_IS_PRIORITY and STRESS_IS_EMPHASIZED

These are some improvements to the german voices, both default and mbrola. It fixes some sounds of phonemes and also pronunciation of words. I also modified the mbrola phoneme and voice files to make them sound more natural. The stressLength setting is that it matches the way espeak deals with the lengths.

This reverts commit 0f55204522.

This breaks using voice files like mb-de5-en. It could also use
named values for the stress rules (e.g. 'first-syllable').

Keyword stressRule in voices.c handles setting langopts->stress_rule.

tr_languages.c still contains the setting for some languages that don't have a language file yet.

Contributes to:
Issue #218, https://github.com/espeak-ng/espeak-ng/issues/218

Contributes to:
Issue #218, https://github.com/espeak-ng/espeak-ng/issues/218

Changes:
language files have a new keyword letterVowel. It can be a latin alphabet from a-z or a hex value (used by bulgarian).
Errors in parsing the values are only reported, nothing is done about them.

About testing:
I haven't noticed any differences in the output audio with letterVowel set or unset in any tested language. The code seems to work and the correct bits seem to be set, but I don't know how to confirm from the final audio.

TODO:
1. Write better documentation in docs/voices.md
2. The code uses new_translator. Should it use translator instead?

See issue #8, https://github.com/espeak-ng/espeak-ng/issues/8

There might still be some ints that could be changed to boolean.

Possible regressions in fifo.c:
int fifo_is_busy() and int fifo_is_command_enabled() used to return an int. Now they return a boolean. This might cause problems on systems where stdbool true/false is something else than 1/0.