dialog [nfc]: Simplify new showErrorDialog interface a bit

Unlike ScaffoldFeatureController which inspired it, this class
doesn't offer a way to control the dialog, so it's not really
a "controller"; just call it DialogStatus.

A private final field with a public getter (of the same type)
is equivalent to just a public final field: nothing can set it,
and any library can get it.

Also generalize the description a bit: we currently use this
only for AlertDialog, but it'd apply equally well for any
dialog box.

Author: gnprice

lib/widgets/content.dart

@@ -1195,7 +1195,7 @@ class GlobalTime extends StatelessWidget {
 }
 
 void _launchUrl(BuildContext context, String urlString) async {
-  DialogStatusController showError(BuildContext context, String? message) {
+  DialogStatus showError(BuildContext context, String? message) {
     return showErrorDialog(context: context,
       title: 'Unable to open link',
       message: [

lib/widgets/dialog.dart

@@ -15,27 +15,26 @@ Widget _dialogActionText(String text) {
   );
 }
 
-/// A wrapper providing access to the status of an [AlertDialog].
+/// Tracks the status of a dialog, in being still open or already closed.
 ///
 /// See also:
 ///  * [showDialog], whose return value this class is intended to wrap.
-class DialogStatusController {
-  const DialogStatusController(this._closed);
+class DialogStatus {
+  const DialogStatus(this.closed);
 
   /// Resolves when the dialog is closed.
-  Future<void> get closed => _closed;
-  final Future<void> _closed;
+  final Future<void> closed;
 }
 
 /// Displays an [AlertDialog] with a dismiss button.
 ///
-/// Returns a [DialogStatusController]. Useful for checking if the dialog has
-/// been closed.
+/// The [DialogStatus.closed] field of the return value can be used
+/// for waiting for the dialog to be closed.
 // This API is inspired by [ScaffoldManager.showSnackBar].  We wrap
-// [showDialog]'s return value, a [Future], inside [DialogStatusController]
+// [showDialog]'s return value, a [Future], inside [DialogStatus]
 // whose documentation can be accessed.  This helps avoid confusion when
 // intepreting the meaning of the [Future].
-DialogStatusController showErrorDialog({
+DialogStatus showErrorDialog({
   required BuildContext context,
   required String title,
   String? message,
@@ -51,7 +50,7 @@ DialogStatusController showErrorDialog({
           onPressed: () => Navigator.pop(context),
           child: _dialogActionText(zulipLocalizations.errorDialogContinue)),
       ]));
-  return DialogStatusController(future);
+  return DialogStatus(future);
 }
 
 void showSuggestedActionDialog({