Widgets: Make a generic postMessage API in WidgetVC

It will be used for https://docs.google.com/document/d/1uPF7XWY_dXTKVKV7jZQ2KmsI19wn9-kFRgQ1tFQP7wQ/edit#.

The Modular postMessage API becomes a specialisation of it.

Riot/ViewController/Widgets/WidgetViewController.h

@@ -20,6 +20,9 @@
 
 /**
  `WidgetViewController` displays widget within a webview.
+
+ It also exposes a generic pipe, the postMessage API, to communicate with the
+ content within the webview, ie the widget (matrix app).
  */
 @interface WidgetViewController : WebViewViewController
 
@@ -31,4 +34,64 @@
  */
 - (instancetype)initWithUrl:(NSString*)widgetUrl forWidget:(Widget*)widget;
 
+/**
+ Display an alert over this controller.
+
+ @param error the error to display.
+ */
+- (void)showErrorAsAlert:(NSError*)error;
+
+
+#pragma mark - postMessage API
+
+/**
+ Callback called when the widget make a postMessage API request.
+
+ This method can be overidden to implement a specific API between the matrix client
+ and widget.
+
+ @param @TODO
+ */
+- (void)onMessage:(NSDictionary*)JSData;
+
+/**
+ Send a boolean response to a request from the widget.
+
+ @param response the response to send.
+ @param @TODO
+ */
+- (void)sendBoolResponse:(BOOL)response toEvent:(NSDictionary*)eventData;
+
+/**
+ Send an integer response to a request from the widget.
+
+ @param response the response to send.
+ @param @TODO
+ */
+- (void)sendIntegerResponse:(NSUInteger)response toEvent:(NSDictionary*)eventData;
+
+/**
+ Send a serialiable object response to a request the widget.
+
+ @param response the response to send.
+ @param @TODO
+ */
+- (void)sendNSObjectResponse:(NSObject*)response toEvent:(NSDictionary*)eventData;
+
+/**
+ Send a message error to a request from the widget.
+
+ @param message the error message.
+ @param @TODO
+ */
+- (void)sendError:(NSString*)message toEvent:(NSDictionary*)eventData;
+
+/**
+ Send a localised message error to a request from the widget.
+
+ @param errorKey the string id of the message error.
+ @param @TODO
+ */
+- (void)sendLocalisedError:(NSString*)errorKey toEvent:(NSDictionary*)eventData;
+
 @end