Simple choice tag naming

[mikir]

Example of the current solution for choices:

zserioChoice.emplace<ConditionValue::ChoiceTag::CHOICE_dateRangeOfYear>(computeDateRangeOfYear(value));

It would be good to simplify naming to be less verbose:

1. To rename ChoiceTag to Tag.
2. To remove prefix CHOICE_ which is used to prevent clashes with the UNDEFINED_CHOICE.
3. Consider to use upper case for the first letters of the tags, e.g. DataRangeOfYear.

Reported by @reinco.