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

Diff for: lib/widgets/content.dart

@@ -1318,7 +1318,7 @@ 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,

Diff for: 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({