compose [nfc]: Pass whole controller down (1/6); _StreamContentInput

For zulip#720, we'd like to add fields to ComposeBoxController and use
them in a bunch of widgets deeper in the compose box, like the
various buttons, without having to add references to them in those
widgets' constructors. With that in mind, pass the whole controller
down, and the widgets can access the fields directly from it. As
suggested by Greg:
  zulip#1090 (comment)

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -442,17 +442,10 @@ class _ContentInputState extends State<_ContentInput> with WidgetsBindingObserve
 
 /// The content input for _StreamComposeBox.
 class _StreamContentInput extends StatefulWidget {
-  const _StreamContentInput({
-    required this.narrow,
-    required this.controller,
-    required this.topicController,
-    required this.focusNode,
-  });
+  const _StreamContentInput({required this.narrow, required this.controller});
 
   final ChannelNarrow narrow;
-  final ComposeContentController controller;
-  final ComposeTopicController topicController;
-  final FocusNode focusNode;
+  final StreamComposeBoxController controller;
 
   @override
   State<_StreamContentInput> createState() => _StreamContentInputState();
@@ -463,29 +456,29 @@ class _StreamContentInputState extends State<_StreamContentInput> {
 
   void _topicChanged() {
     setState(() {
-      _topicTextNormalized = widget.topicController.textNormalized;
+      _topicTextNormalized = widget.controller.topic.textNormalized;
     });
   }
 
   @override
   void initState() {
     super.initState();
-    _topicTextNormalized = widget.topicController.textNormalized;
-    widget.topicController.addListener(_topicChanged);
+    _topicTextNormalized = widget.controller.topic.textNormalized;
+    widget.controller.topic.addListener(_topicChanged);
   }
 
   @override
   void didUpdateWidget(covariant _StreamContentInput oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (widget.topicController != oldWidget.topicController) {
-      oldWidget.topicController.removeListener(_topicChanged);
-      widget.topicController.addListener(_topicChanged);
+    if (widget.controller.topic != oldWidget.controller.topic) {
+      oldWidget.controller.topic.removeListener(_topicChanged);
+      widget.controller.topic.addListener(_topicChanged);
     }
   }
 
   @override
   void dispose() {
-    widget.topicController.removeListener(_topicChanged);
+    widget.controller.topic.removeListener(_topicChanged);
     super.dispose();
   }
 
@@ -498,8 +491,8 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     return _ContentInput(
       narrow: widget.narrow,
       destination: TopicNarrow(widget.narrow.streamId, _topicTextNormalized),
-      controller: widget.controller,
-      focusNode: widget.focusNode,
+      controller: widget.controller.content,
+      focusNode: widget.controller.contentFocusNode,
       hintText: zulipLocalizations.composeBoxChannelContentHint(streamName, _topicTextNormalized));
   }
 }
@@ -1156,9 +1149,7 @@ class _StreamComposeBoxBody extends _ComposeBoxBody {
 
   @override Widget contentInput() => _StreamContentInput(
     narrow: narrow,
-    topicController: controller.topic,
-    controller: controller.content,
-    focusNode: controller.contentFocusNode,
+    controller: controller,
   );
 
   @override Widget sendButton() => _SendButton(