compose [nfc]: s/_ComposeBoxLayout/_ComposeBoxBody/

This widget is already doing more than layout (sizing and
positioning its inputs on the screen): it creates the attachment
buttons and styles their touch feedback, and it defeats an unwanted
border that its `topicInput` and `contentInput` params might
otherwise create.

I chose the name "body" to distinguish this from some other elements
that will need to share the compose-box surface, for the UI part of
issue zulip#720: an error banner and a linear progress indicator.

Then also add "body" to the names _StreamComposeBox and
_FixedDestinationComposeBox. Since the recent commits that
simplified those, they're really just thin, stateless wrappers that
configure a _ComposeBoxBody.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1030,8 +1030,8 @@ class _ComposeBoxContainer extends StatelessWidget {
   }
 }
 
-class _ComposeBoxLayout extends StatelessWidget {
-  const _ComposeBoxLayout({
+class _ComposeBoxBody extends StatelessWidget {
+  const _ComposeBoxBody({
     required this.topicInput,
     required this.contentInput,
     required this.sendButton,
@@ -1134,8 +1134,8 @@ class FixedDestinationComposeBoxController extends ComposeBoxController {}
 ///
 /// This offers a text input for the topic to send to,
 /// in addition to a text input for the message content.
-class _StreamComposeBox extends StatelessWidget {
-  const _StreamComposeBox({required this.narrow, required this.controller});
+class _StreamComposeBoxBody extends StatelessWidget {
+  const _StreamComposeBoxBody({required this.narrow, required this.controller});
 
   /// The narrow on view in the message list.
   final ChannelNarrow narrow;
@@ -1148,7 +1148,7 @@ class _StreamComposeBox extends StatelessWidget {
       :topic, :content, :topicFocusNode, :contentFocusNode,
     ) = controller;
 
-    return _ComposeBoxLayout(
+    return _ComposeBoxBody(
       contentController: content,
       contentFocusNode: contentFocusNode,
       topicInput: _TopicInput(
@@ -1193,8 +1193,8 @@ class _ErrorBanner extends StatelessWidget {
   }
 }
 
-class _FixedDestinationComposeBox extends StatelessWidget {
-  const _FixedDestinationComposeBox({required this.narrow, required this.controller});
+class _FixedDestinationComposeBoxBody extends StatelessWidget {
+  const _FixedDestinationComposeBoxBody({required this.narrow, required this.controller});
 
   final SendableNarrow narrow;
 
@@ -1206,7 +1206,7 @@ class _FixedDestinationComposeBox extends StatelessWidget {
       :content, :contentFocusNode,
     ) = controller;
 
-    return _ComposeBoxLayout(
+    return _ComposeBoxBody(
       contentController: content,
       contentFocusNode: contentFocusNode,
       topicInput: null,
@@ -1316,15 +1316,15 @@ class _ComposeBoxState extends State<ComposeBox> implements ComposeBoxState {
     switch (narrow) {
       case ChannelNarrow():
         assert(_controller is StreamComposeBoxController);
-        return _StreamComposeBox(controller: (_controller as StreamComposeBoxController),
+        return _StreamComposeBoxBody(controller: (_controller as StreamComposeBoxController),
           narrow: narrow);
       case TopicNarrow():
         assert(_controller is FixedDestinationComposeBoxController);
-        return _FixedDestinationComposeBox(controller: (_controller as FixedDestinationComposeBoxController),
+        return _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
           narrow: narrow);
       case DmNarrow():
         assert(_controller is FixedDestinationComposeBoxController);
-        return _FixedDestinationComposeBox(controller: (_controller as FixedDestinationComposeBoxController),
+        return _FixedDestinationComposeBoxBody(controller: (_controller as FixedDestinationComposeBoxController),
           narrow: narrow);
       case CombinedFeedNarrow():
       case MentionsNarrow():