firmware: add capability to short all inputs (via serial command)

firmware/Include/main.h

@@ -55,7 +55,10 @@ void Error_Handler(void);
 
 void set_input(uint8_t channel);
 void set_attenuation(uint8_t left, uint8_t right);
-void set_mute(uint8_t mute);
+void set_mute(uint8_t mute); //< tries to switch to muted ui state
+int is_muted(); //< returns whether the ui is muted
+void set_short(uint8_t shorted); //< tries to switch to shorted ui state
+int is_shorted();
 
 /* USER CODE BEGIN EFP */