Support "Centralize most color definitions" (#50)

Support changes in InfiniTimeOrg/InfiniTime#1258

The file `lv_pinetime_theme.c` is renamed to `InfiniTimeTheme.cpp`.
Need to pick up that change in the simulator to compile with the new cpp
file if it exists.

Furthermore use the new `InfiniTimeTheme.h` header in simulator files.

Fixes: https://github.com/InfiniTimeOrg/InfiniSim/issues/49

sim/displayapp/LittleVgl.cpp

@@ -1,5 +1,9 @@
 #include "displayapp/LittleVgl.h"
+#if defined(INFINITIME_THEME_CPP)
+#include "displayapp/InfiniTimeTheme.h"
+#else
 #include "displayapp/lv_pinetime_theme.h"
+#endif
 
 #include <FreeRTOS.h>
 #include <task.h>