Wizard battery monitor step doesn't pick up existing values and disables monitor

#### Issue details

If I rerun the wizard the _Battery Monitor_ step fails to pickup the current values and if you don't set them again the battery monitor will end up disabled.

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Let's run through the steps. Here I've set the various values in the _Battery Monitor_ step when running the wizard for the first time:

When I exit out of the wizard and check the _Battery Monitor_ section I see the _Monitor_, _Sensor_ and _APM Ver_ correctly set as expected:

If I restart the wizard and step forward to the _Battery Monitor_ step I see that it's retrieved the the battery size (3700) correctly but not the autopilot version or sensor:

If I do nothing (i.e. don't press _Next_ or update any of the fields) and simply exit the wizard and return to the _Battery Monitor_ section I find that the _Monitor_ value has switched from _Voltage and Current_ to _Disabled_:

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While I'm here - thanks Michael for creating an amazing piece of software :smile: And sorry that the first issue that I logged against MP (#1620) wasn't properly researched. And also apologies for not digging into the code to look for a fix - I try to contribute to the OS projects that I benefit from but I'm more a Linux / Java person so I don't have a nice C# setup for investigating deeper.

#### Version
1.3.49

#### Platform
[ ] All
[ ] AntennaTracker
[ * ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
X quad

#### Hardware type
Pixhawk 1

Reproduce the issue in the Mission Planner wizard using the Battery Monitor step and the Copter configuration described in the report. Trace how the existing battery size, autopilot version, sensor, and monitor values are loaded and saved; done means rerunning and exiting the wizard preserves all four configured values instead of disabling monitoring.

Written by the indexing model from the issue text.