PX4 flight modes support

[thedevleon]

Using Tower (which uses this drone-kit) connects to my Pixhawk running PX4 just fine, but flight modes are not displayed, nor can be changed. It just says "Unknown".

Can PX4 flight modes be added? Let me explain how it works:
Basically, PX4 sends it's flight mode in a custom data format px4_custom_mode via the custom_mode variable in the mavlink heartbeat message.

For more info, see how it's sent and how it's encoded.