Widgets: Make sure we have a scalar token before opening one (#1723).

Make the build of the true widget URL asynchronous and build it only when necessary.

Riot/ViewController/Widgets/WidgetViewController.h

@@ -26,8 +26,9 @@
 /**
  Init 'WidgetViewController' instance with a widget.
 
+ @param widgetUrl the formatted widget url.
  @param widget the widget to open.
  */
-- (instancetype)initForWidget:(Widget*)widget;
+- (instancetype)initWithUrl:(NSString*)widgetUrl forWidget:(Widget*)widget;
 
 @end