Add ability to reset parameter to its default

Now, you can reset a parameter to its default value by entering the string "d",
"de", "def", ..., or "default" into the second input field. Also, the returned
status now displays a string representation of the actual new value rather than
the exact string that the user entered, so if you type ".01", you will now see
"0.01" rather than ".01" as before.

I was thinking about adding another special way of resettig ALL parameters to
their default values, but I don't think I will bother. I don't think anyone
would actually need to do that.