Computed Fields optimized

The computed fields definition and output now work as planned.  The
user can click the get buttons from the grid to place data into the
definition text field.  The user can also place custom data into the
definition text field.  Elements in the definition are separated by
semi-colons.
Here's an example:
{"field":"Port Nominal Voltage","property":"value"};{"field":"Port
Nominal Voltage","property":"unit"};", up to ";{"field":"Port Current
Max","property":"value"};{"field":"Port Current Max","property":"unit"}

The result is:
5V, up to 1.5A

(The numbers and units are collected and matched from the database).

The definition is still not yet added to the database.