Major revision to NWR transcription

## Design changes
* Procedures are used whenever possible to contain behavior within meaningful subroutines. For example, `@transcribe_segment(...)` checks whether the segment is a vowel and calls either `@transcribe_consonant(...)` or `@transcribe_vowel(...)` procedure. The consonant transcription procedure in turn calls `@transcribe_cons_manner(...)`, `@transcribe_cons_symbol(...)`, `@transcribe_cons_place_voice(...)`, and `@score_consonant(...)`. 
* Procedures are also used to contain or (hide) related variables in a common namespace. For example, `@nwr_trans_textgrid(...)` defines all the important variables needed to define and describe the transcription textgrid -- for example, its filepath (locally `.filepath$` or globally `nwr_trans_textgrid.filepath$`).
* Transcription steps are managed by an event loop with 7 nodes: transcribe T1, score T1, transcribe T2, score T2, transcribe prosody, score prosody, and save results.
* Start-up wizard now re-uses code from segmentation start-up.
* Functionality is broken up over multiple files.

## Transcription changes
* CV trial type has been generalized so VC and CV trials are now supported.
* Prosody scoring now based on insertion and deletion of segments.
* Diphthongs scored for frontness and backness based on the first vowel.
* /l/ and /r/ added as "voiced" "alveolar" "glides"

## Other changes
* Minimum version of praat is now enforced. Praat commands that uses `Command... [args]` notation have been replaced with the recent `Command: [args]` notation.
* Multistep transcription wizards (consonants) support a back-button.