HUD Mavlink message

#### Issue details
What MAVlink message (and field) modifies the current flight mode on the Mission Planner HUD?
(# 15 on the figure below)
I have sent a Heartbeat message with the base_mode field equal to MAV_MODE_FLAG_SAFETY_ARMED=128 enum, but the HUD only displays UNKNOWN.

Also, what modifies the battery display on HUD (# 10 on the figure)?
For me it is not visible, although I send sys_status and battery_status messages with dummy values.

#### Version
Latest MP

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

#### Airframe type
fixed wing

#### Hardware type
Custom Raspberry PI based hardware


The issue names the Mission Planner HUD and MAVLink HEARTBEAT, SYS_STATUS, and BATTERY_STATUS messages, but provides no source file or test. Start by locating the HUD message-handling entry point and reproducing the Plane case with the stated dummy messages. Done means documenting or correcting which fields drive the flight-mode and battery displays.

Written by the indexing model from the issue text.