Shows horizontal/vertical scroll indicators

* Vehicle selection must be global control
* Disconnect controls must be associated with Vehicle

Made QGCCorePlugin derive from QGCToolBox and expose everything through the same mechanism as everything else.

Also removed intermediate param load retry message

Moving source files into Vehicles as it is a more coherent place.

Forcing QGC to use Mavlink V2 if vehicle supports it.

This makes it available on mobile builds

Too buggy to leave in for now

Also added new FirmwarePlugin::brandImage support

No signing support yet

_finalize methods need information which is not available if all you
have is a mavlink_message_t

This reverts commit ae976049ca7b4818616e6b8be07a028fc7dfc26f.

Includes ability to load params