Rewrite of start-up wizard event loop

The start-up wizard has been substantially simplified. 

* The code for each step of the start-up process is now encapsulated in a procedure. 
* These procedures don't have any hard-coded special knowledge about the previous or next steps in the wizards. Instead, each of these procedures produces a `.result_node$` value which tells whether the wizard should go backward, forwards or abort. This interface allows these procedures to be recycled for other tasks/scripts.
* Each of these procedure are supported by another `log_` procedure which prints out info-lines about the values of the variables inside the procedure. This logging is toggled by a global variable `debug`. 

Below are some lines from the logging:

```
Node: initials

---- log_initials() ----
Exit Status: next
derived values: 
	.initials$: TM
	.drive$: L:/
	.audio_drive$: L:/

---- log_testwave() ----
Exit Status: next
derived values: 
	.task$: RealWordRep
	.testwave$: TimePoint1
```

By packing some lines of code away in subroutines, variables created in those lines of code are no longer part of the script's global namespace. I've made an effort to re-introduce some of those variables back into the global namespace with lines like: 

```
# [global variable] = [procedure name].[matching variable in procedure]
drive$ = startup_initials.drive$
audio_drive$ = startup_initials.audio_drive$
task$ = startup_testwave.task$
testwave$ = startup_testwave.testwave$
segmenters_initials$ = startup_initials.initials$
```

But if there is a variable I haven't yet put into the global namespace, these variables will not be found and the script will crash. I will have some user's test this version of the script.
bug fixes, light refactors

* Attach wordlist variables to global namespace
* Adapting the new Praat menu command syntax "Command: arg" (instead of "Command... 'arg'")
added TargetPromptMissing as context option
Merge branch 'master' of https://github.com/tjmahr/Segmentation into tjmahr-master

Conflicts:
	Segmentation.praat