compose [nfc]: Extract _ComposeButton

Not all buttons are about uploading files, while most of the
buttons on the compose box share the same design.  Extract that
part for later use.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/compose_box.dart

@@ -978,14 +978,32 @@ Future<void> _uploadFiles({
   }
 }
 
-abstract class _AttachUploadsButton extends StatelessWidget {
-  const _AttachUploadsButton({required this.controller});
+abstract class _ComposeButton extends StatelessWidget {
+  const _ComposeButton({required this.controller});
 
   final ComposeBoxController controller;
 
   IconData get icon;
   String tooltip(ZulipLocalizations zulipLocalizations);
 
+  void handlePress(BuildContext context);
+
+  @override
+  Widget build(BuildContext context) {
+    final designVariables = DesignVariables.of(context);
+    final zulipLocalizations = ZulipLocalizations.of(context);
+    return SizedBox(
+      width: _composeButtonSize,
+      child: IconButton(
+        icon: Icon(icon, color: designVariables.foreground.withFadedAlpha(0.5)),
+        tooltip: tooltip(zulipLocalizations),
+        onPressed: () => handlePress(context)));
+  }
+}
+
+abstract class _AttachUploadsButton extends _ComposeButton {
+  const _AttachUploadsButton({required super.controller});
+
   /// Request files from the user, in the way specific to this upload type.
   ///
   /// Subclasses should manage the interaction completely, e.g., by catching and
@@ -995,7 +1013,8 @@ abstract class _AttachUploadsButton extends StatelessWidget {
   /// return an empty [Iterable] after showing user feedback as appropriate.
   Future<Iterable<_File>> getFiles(BuildContext context);
 
-  void _handlePress(BuildContext context) async {
+  @override
+  void handlePress(BuildContext context) async {
     final files = await getFiles(context);
     if (files.isEmpty) {
       return; // Nothing to do (getFiles handles user feedback)
@@ -1013,18 +1032,6 @@ abstract class _AttachUploadsButton extends StatelessWidget {
       contentFocusNode: controller.contentFocusNode,
       files: files);
   }
-
-  @override
-  Widget build(BuildContext context) {
-    final designVariables = DesignVariables.of(context);
-    final zulipLocalizations = ZulipLocalizations.of(context);
-    return SizedBox(
-      width: _composeButtonSize,
-      child: IconButton(
-        icon: Icon(icon, color: designVariables.foreground.withFadedAlpha(0.5)),
-        tooltip: tooltip(zulipLocalizations),
-        onPressed: () => _handlePress(context)));
-  }
 }
 
 Future<Iterable<_File>> _getFilePickerFiles(BuildContext context, FileType type) async {