Add support for more than one system, and subsdevices (a part of MAVlink)

[AndKe]

Today QGC can only control one system/one AP.
Connecting to Autopilot (SYS_ID 1 , and COMP_ID 1) - I see and can edit it's parameters.
In MAVproxy/MP etc, I can also select(it is autodetected by heartbeat) SYS_ID 1 . COMP_ID 191 - which is another onboard system, and see/modify it's parameters.

• What problem does it solve?
• What does it do?

control more than one/system, allows the control of subsystems

• What flight stacks must it work with? (All, PX4, ArduPilot)
  it is universal for all MAVLink systems.

• Relevant vehicle types? (All, multirotor, fixed-wing, VTOL, submarine, etc.)
  all

• Are there other systems that have this feature?
  MissionPlanner, MAVProxy, AP2

• What communication/integration standards does the feature rely on (e.g. MAVLink commands etc.)
  Simply add systems when seeing heartbeat packets from them, sort by SYS_ID, COMP_ID