widgets/content: Add MessageInheritedWidget to provide message simply

A lot of different widgets, some pretty deeply nested, go into
rendering a message's content. This way each of those widgets can
get data from the relevant Message object as needed, without the
need for passing it all the way down, from widget params to widget
params.

Author: chrisbobbe

lib/widgets/content.dart

@@ -22,7 +22,24 @@ class MessageContent extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return BlockContentList(nodes: content.nodes);
+    return InheritedMessage(message: message, child: BlockContentList(nodes: content.nodes));
+  }
+}
+
+/// Provides access to [message].
+class InheritedMessage extends InheritedWidget {
+  const InheritedMessage({super.key, required this.message, required super.child});
+
+  final Message message;
+
+  @override
+  bool updateShouldNotify(covariant InheritedMessage oldWidget) =>
+    !identical(oldWidget.message, message);
+
+  static InheritedMessage of(BuildContext context) {
+    final widget = context.dependOnInheritedWidgetOfExactType<InheritedMessage>();
+    assert(widget != null, 'No InheritedMessage ancestor');
+    return widget!;
   }
 }