compose [nfc]: Convert _TopicInput to a StatefulWidget

We will shortly start adding listeners for this.

Author: PIG208

lib/widgets/compose_box.dart

@@ -671,12 +671,17 @@ class _StreamContentInputState extends State<_StreamContentInput> {
   }
 }
 
-class _TopicInput extends StatelessWidget {
+class _TopicInput extends StatefulWidget {
   const _TopicInput({required this.streamId, required this.controller});
 
   final int streamId;
   final StreamComposeBoxController controller;
 
+  @override
+  State<_TopicInput> createState() => _TopicInputState();
+}
+
+class _TopicInputState extends State<_TopicInput> {
   @override
   Widget build(BuildContext context) {
     final zulipLocalizations = ZulipLocalizations.of(context);
@@ -688,18 +693,18 @@ class _TopicInput extends StatelessWidget {
     ).merge(weightVariableTextStyle(context, wght: 600));
 
     return TopicAutocomplete(
-      streamId: streamId,
-      controller: controller.topic,
-      focusNode: controller.topicFocusNode,
-      contentFocusNode: controller.contentFocusNode,
+      streamId: widget.streamId,
+      controller: widget.controller.topic,
+      focusNode: widget.controller.topicFocusNode,
+      contentFocusNode: widget.controller.contentFocusNode,
       fieldViewBuilder: (context) => Container(
         padding: const EdgeInsets.only(top: 10, bottom: 9),
         decoration: BoxDecoration(border: Border(bottom: BorderSide(
           width: 1,
           color: designVariables.foreground.withFadedAlpha(0.2)))),
         child: TextField(
-          controller: controller.topic,
-          focusNode: controller.topicFocusNode,
+          controller: widget.controller.topic,
+          focusNode: widget.controller.topicFocusNode,
           textInputAction: TextInputAction.next,
           style: topicTextStyle,
           decoration: InputDecoration(