Split SystemMonitor into h and cpp file and move the logging code of the
`Process` function into the cpp file.
Depending of the `configUSE_TRACE_FACILITY` define from
`src/FreeRTOSConfig.h` create either a "FreeRtosMonitor" or a
"DummyMonitor".
Make the `Process()` function non-const, as the FreeRtosMonitor changes
the member variable `lastTick`.
In `SystemTask.h` we then only need to use `SystemMonitor`, without
knowledge of the `configUSE_TRACE_FACILITY` define.
