LibIPC: Allow opt-in UTF-8 validation on message parameters

You can now mark String message parameters with the [UTF8] attribute.
This will cause the generated decoder to perform UTF-8 validation and
reject the message if the given parameter is not a valid UTF-8 string.

This frees up the receiving side from having to do this validation at
a higher level.

Services/Clipboard/ClipboardClient.ipc

@@ -1,4 +1,4 @@
 endpoint ClipboardClient = 804
 {
-    ClipboardDataChanged(String mime_type) =|
+    ClipboardDataChanged([UTF8] String mime_type) =|
 }