widgets: Make sure the customValue and customChoice still work

[tahini]

And streamline and document their behavior, which is currently spread in many places.

Currently, only the 'radio' input of the 'question' widget supports a custom choice and value, through the 'customPath, 'customChoice', customLabel' and 'addCustom' fields. But the customPath is used and sent to many functions, un-necessarily. There's a a lot of code associated with it and it's not certain that it really still works (and how!)

1- Should we support the custom value with other widgets? (for example, select, checkbox, etc)
2- Document the 4 parameters involved in the behavior and add tests for them
3- Make sure the widgets send the correct data and it is properly handled by the widget preparation function.

[samuel-duhaime]

@tahini
For your point 1. Yes, it will be useful to have that on the checkbox and maybe select. I would have used it a few times in my surveys.

[tahini]

Also, the widget preparation function uses a customDefaultValue that is not defined for any widgets. Is it needed? If so, we need to define it in the configs

[samuel-duhaime]

@tahini
We sometimes use defaultValue, for example, when we hide some questions but know what the answer would be. However, I don't remember using customDefaultValue anywhere.

@kaligrafy Any idea ?