GearyWebExtension: Add support for sending messages from JS to client

Define a vala-backed JS class in the extension and make that available
to pages when they are registered. Add some helper JS to PageState for
defining message sending functions. Listen for these in
Components.WebView and dispatch to the registered callback for it.

ui/components-web-view.js

@@ -200,3 +200,12 @@ PageState.prototype = {
         throw this.testResult;
     }
 };
+
+let MessageSender = function(name) {
+    return function() {
+        // Since typeof(arguments) == 'object', convert to an array so
+        // that Components.WebView.MessageCallback callbacks get
+        // arrays or tuples rather than dicts as arguments
+        _GearyWebExtension.send(name, Array.from(arguments));
+    };
+};