[host] windows: detect whether screensaver is disabled in the guest

This will allow us to add an option to disable the screensaver on the client
when an application in the guest requests it. This behaviour may be useful
when the guest is doing media playback.

host/include/interface/platform.h

@@ -38,3 +38,5 @@ void app_quit();
 // these must be implemented for each OS
 const char * os_getExecutable();
 const char * os_getDataPath();
+
+bool os_blockScreensaver();