SystemMonitor: implement FreeRtosMonitor only if trace facility is set

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.

src/CMakeLists.txt

@@ -516,6 +516,7 @@ list(APPEND SOURCE_FILES
         displayapp/lv_pinetime_theme.c
 
         systemtask/SystemTask.cpp
+        systemtask/SystemMonitor.cpp
         drivers/TwiMaster.cpp
 
         heartratetask/HeartRateTask.cpp
@@ -577,6 +578,7 @@ list(APPEND RECOVERY_SOURCE_FILES
         FreeRTOS/port_cmsis.c
 
         systemtask/SystemTask.cpp
+        systemtask/SystemMonitor.cpp
         drivers/TwiMaster.cpp
         components/gfx/Gfx.cpp
         components/rle/RleDecoder.cpp