FGData Merge Request #60: Fix saving/loading the transponder code to/from the property-tree. (merged)

When opening the Radio Frequencies dialog the transponder code is loaded from /instrumentation/transponder/id-code. There are two issues here:

1. The combo-boxes for the digits are filled with float values instead of int values. A code of 1234 will be shown as 1.2 - 2.3 - 3.4 - 4.0.

2. The transponder code entered in the dialog is currently not saved to the above mentioned property.

This merge request fixes both.