feat: Add support for SpaceMouse mod (6D input knob) (#1853)

Use `-DSPACEMOUSE=YES` CMake option to activate on Horus color targets.
Co-authored-by: Niel Horne (elecpower), Raphael Coeffic (raphaelcoeffic) and Peter Feerick (pfeerick).

radio/src/opentx.cpp

@@ -547,6 +547,11 @@ bool inputsMoved()
     sum += getValue(MIXSRC_TILT_X+i) >> INAC_STICKS_SHIFT;
 #endif
 
+#if defined(SPACEMOUSE)
+  for (uint8_t i=0; i<(MIXSRC_LAST_SPACEMOUSE - MIXSRC_FIRST_SPACEMOUSE); i++)
+    sum += get_spacemouse_value(i) >> INAC_STICKS_SHIFT;
+#endif
+
   if (abs((int8_t)(sum-inactivity.sum)) > 1) {
     inactivity.sum = sum;
     return true;