Timer: Use explicit re-initialization on file load

Before this the timer API was relying on using a callback API to do
initialization when new file is loaded. This isn't how rest of Blender
works and it gets in a way because callbacks API is to be move to the
BKE level.

Use explicit call to timer API from where the file is loaded.

source/blender/blenlib/BLI_timer.h

@@ -50,4 +50,8 @@ void BLI_timer_execute(void);
 
 void BLI_timer_free(void);
 
+/* This function is to be called next to BLI_CB_EVT_LOAD_PRE, to make sure the module
+ * is properly configured for the new file. */
+void BLI_timer_on_file_load(void);
+
 #endif /* __BLI_TIMER_H__ */