RfC: Add a method to inject RC RSSI data from external source

Hi all,

This is a request for comments regarding a small change I would like to introduce in Vehicle.cc.

Right now, the only way to inform the UI about the RC RSSI is via the MAVLINK RC_CHANNEL message. This, however, does not work well when we receive RSSI data using some other method, for example out-of-band connectivity between the QGC and the RC controller firmware. One could hack the “RCRSSIIndicator.qml” or build a new version of one, but I would rather leave everything as-is.

In my view, the least intrusive method would be something like:

void Vehicle::setRCRSSIExternal(uint8_t rssi)
    emit remoteControlRSSIChanged(rssi);

Any downsides to this?