Change external-compass checkbox to combo box

The COMPASS_EXTERNAL settings now support a value of 2 (for forced external) -- see ArduPilot PR [4011](https://github.com/ArduPilot/ardupilot/pull/4011). It'd be good if the external-compass option on the "Compass" page could be changed from a check box to a pull-down combo-box with the available options (0:Internal, 1:External, 2:Forced External).

--ET

Start at the Mission Planner "Compass" page and trace how the COMPASS_EXTERNAL setting is represented by the current checkbox. Replace that control with a combo box offering 0: Internal, 1: External, and 2: Forced External, and verify that selecting each option reads and stores the corresponding value.

Written by the indexing model from the issue text.