Add support for BtnMultipress

[lucasdemarchi]

Like BtnStep, but the next steps are always absolute rather than relative to the current state. This allows to mentally map then number of presses to the value sent in the channel, e.g. for mode changes, like 1 press: stabilize, 2 presses: loiter; 3 presses: RTH

The mode itself is configured in the flight stack; we only send the different step in the RCChannel