Implemented groupable defvars.
* Added the member `__group` with corresponding getter `LocaleString group()` and setter `this_program set_group(LocaleString name)`.
* Also changed all `void` to return the object being called to enable chained calls.
* emit#module-variables checks if any groups are defined in the variable list, and if so group those together. If an item is a group the mapping will contain the members `is-group`, `group (the name of the group)` and `vars (the list of variables in the group)`.
* The tag definition `<cfg-variables>` handles the emission of grouped defvars.