MAVLink v2 compatibility and features

[wangyeee]

Currently only mavlink v1 devices have been tested with the gcs code. No mavlink v2 devices have been tested. And the gcs needs to support new features in mavlink v2 such as message signing, etc.

[wangyeee]

Initial tests passed with custom board running AutoQuad firmware from wangyeee/aq_flight_control@183118a

[wangyeee]

Message signing setup window is added in 0f134af.

[wangyeee]

Support for SETUP_SIGNING message is in a79832a