dialog [nfc]: Generalize to ModalStatus, renaming DialogStatus

We'll soon use this for modal bottom sheets too, aka action sheets.

Arguably that could mean it belongs in some other file, not specific
to dialogs nor bottom sheets.  But there isn't an obvious other file
for it, and I think this is as good a home as any.

Author: gnprice

lib/widgets/content.dart

@@ -1214,7 +1214,7 @@ class GlobalTime extends StatelessWidget {
   final GlobalTimeNode node;
   final TextStyle ambientTextStyle;
 
-  static final _dateFormat = DateFormat('EEE, MMM d, y, h:mm a'); // TODO(i18n): localize date
+  static final _dateFormat = DateFormat('EEE, MMM d, y, h:mm a'); // TODO(intl): localize date
 
   @override
   Widget build(BuildContext context) {
@@ -1318,12 +1318,12 @@ class MessageTableCell extends StatelessWidget {
 }
 
 void _launchUrl(BuildContext context, String urlString) async {
-  DialogStatus showError(BuildContext context, String? message) {
+  ModalStatus showError(BuildContext context, String? message) {
     final zulipLocalizations = ZulipLocalizations.of(context);
     return showErrorDialog(context: context,
-      title: zulipLocalizations.errorCouldNotOpenLinkTitle,
+      title: 'Unable to open link',
       message: [
-        zulipLocalizations.errorCouldNotOpenLink(urlString),
+        'Link could not be opened: $urlString',
         if (message != null) message,
       ].join("\n\n"));
   }
@@ -1571,7 +1571,6 @@ InlineSpan _errorUnimplemented(UnimplementedNode node, {required BuildContext co
   // because release mode isn't yet about general users but developer demos,
   // and we want to keep the demos honest.
   // TODO(#194) think through UX for general release
-  // TODO(#1285) translate this
   final htmlNode = node.htmlNode;
   if (htmlNode is dom.Element) {
     return TextSpan(children: [

lib/widgets/dialog.dart

@@ -16,26 +16,28 @@ Widget _dialogActionText(String text) {
   );
 }
 
-/// Tracks the status of a dialog, in being still open or already closed.
+/// Tracks the status of a modal dialog or modal bottom sheet,
+/// in being still open or already closed.
 ///
 /// See also:
-///  * [showDialog], whose return value this class is intended to wrap.
-class DialogStatus {
-  const DialogStatus(this.closed);
+///  * [showDialog] and [showModalBottomSheet], whose return values
+///    this class is intended to wrap.
+class ModalStatus {
+  const ModalStatus(this.closed);
 
-  /// Resolves when the dialog is closed.
+  /// Resolves when the dialog or bottom sheet is closed.
   final Future<void> closed;
 }
 
 /// Displays an [AlertDialog] with a dismiss button.
 ///
-/// The [DialogStatus.closed] field of the return value can be used
+/// The [ModalStatus.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 [DialogStatus]
+// [showDialog]'s return value, a [Future], inside [ModalStatus]
 // whose documentation can be accessed.  This helps avoid confusion when
 // intepreting the meaning of the [Future].
-DialogStatus showErrorDialog({
+ModalStatus showErrorDialog({
   required BuildContext context,
   required String title,
   String? message,
@@ -51,7 +53,7 @@ DialogStatus showErrorDialog({
           onPressed: () => Navigator.pop(context),
           child: _dialogActionText(zulipLocalizations.errorDialogContinue)),
       ]));
-  return DialogStatus(future);
+  return ModalStatus(future);
 }
 
 void showSuggestedActionDialog({