Refactor the emulation mode

[denis-stepanov]

Currently the transmitter emulation mode is just a bunch of alternative code dispersed across the project and guarded by #define DS_DEVBOARD, mostly in the Receiver class. It should be possible to make this part more C++-like by making a dedicated Receiver class for this code. An option to investigate is bringing emulation down on the level of serial port (this would spare a second Receiver class).