oovbaapi: implement Application.GetOperatingSystem

Without version number of the system for now.

Documentation:
https://msdn.microsoft.com/en-us/vba/excel-vba/articles/application-operatingsystem-property-excel

Change-Id: I0c76ea3909ecaa3896823136b3a3ebf2a2fdb4d1
Reviewed-on: https://gerrit.libreoffice.org/39383
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Bunth <btomi96@gmail.com>

sc/source/ui/vba/vbaapplication.cxx

@@ -917,9 +917,18 @@ ScVbaApplication::getPathSeparator()
 OUString SAL_CALL
 ScVbaApplication::getOperatingSystem()
 {
-    // TODO implement
-    SAL_WARN("sc", "not implemented");
-    return OUString();
+    // TODO Solution should contain the version number of the operating system
+    // too.
+#if   defined(_WIN32)
+        return OUString("Windows");
+#elif defined(MACOSX)
+        return OUString("Macintosh");
+#elif defined(UNX)
+        // M. Office is not available on Unix systems, so it is not documented.
+        return OUString("Unix");
+#else
+        return OUString("Unknown");
+#endif
 }
 
 // Helpers for Intersect and Union