[FIX] RT-1227: Client should parse "address?dt=xxx"

https://www.bountysource.com/issues/2842566-client-should-parse-address-dt-xxx

Now works correctly.
The destination tag will always be filled in from this even if a contact exists with a similar name.
Contacts will now only overwrite the destination tag if it is blank or if the field is hidden.
This prevents contacts from overwriting the destination tag provided by the ?dt= or manually by the user.

-Joseph LoManto (aka YourNerdyJoe)

The original reasoning behind this was "show dt field after writing the contact's dt."
However, the program works the same with and without that line since it shows the field when dt is written to.
I put it in when I was learning the behavior of the program and forgot to remove since it didn't affect the program's behavior.