- merge FGDialog::{update,apply}Value{,s} ... there's really no need to have

- merge FGDialog::{update,apply}Value{,s} ... there's really no need to have
  two almost identical functions for these methods. It only forces to repeat
  the redundancy for every small change to either.
- abstract out generation and destruction of plib string arrays
- abstract out generation of lists from <value> children