Fix window does not show modified when editing subtitles

As reported here:
https://forum.shotcut.org/t/beta-version-24-08-now-available-to-test/45307/35

src/mainwindow.cpp

@@ -688,6 +688,7 @@ void MainWindow::setupAndConnectDocks()
             SLOT(onAddAllToTimeline(Mlt::Playlist *, bool, bool)));
     connect(m_subtitlesDock, SIGNAL(createOrEditFilterOnOutput(Mlt::Filter *, const QStringList &)),
             SLOT(onCreateOrEditFilterOnOutput(Mlt::Filter *, const QStringList &)));
+    connect(m_timelineDock->subtitlesModel(), SIGNAL(modified()), this, SLOT(onSubtitleModified()));
 
     addDockWidget(Qt::LeftDockWidgetArea, m_propertiesDock);
     addDockWidget(Qt::RightDockWidgetArea, m_recentDock);
@@ -3165,6 +3166,11 @@ void MainWindow::onNoteModified()
     setWindowModified(true);
 }
 
+void MainWindow::onSubtitleModified()
+{
+    setWindowModified(true);
+}
+
 void MainWindow::onCutModified()
 {
     if (!playlist() && !multitrack()) {

src/mainwindow.h

@@ -282,6 +282,7 @@ private slots:
     void onMultitrackModified();
     void onMultitrackDurationChanged();
     void onNoteModified();
+    void onSubtitleModified();
     void onCutModified();
     void onProducerModified();
     void onFilterModelChanged();

src/models/subtitlesmodel.cpp

@@ -190,6 +190,7 @@ void SubtitlesModel::commitToFeed(int trackIndex)
             feedFilterIndex++;
         }
     }
+    emit modified();
 }
 
 void SubtitlesModel::addTrack(SubtitlesModel::SubtitleTrack &track)
@@ -499,6 +500,7 @@ void SubtitlesModel::doInsertTrack(const SubtitlesModel::SubtitleTrack &track, i
     m_producer->move_filter(m_producer->filter_count() - 1, filterIndex);
     endInsertRows();
     emit tracksChanged(m_tracks.size());
+    emit modified();
 }
 
 void SubtitlesModel::doRemoveTrack(int trackIndex)
@@ -526,6 +528,7 @@ void SubtitlesModel::doRemoveTrack(int trackIndex)
     }
     endRemoveRows();
     emit tracksChanged(m_tracks.size());
+    emit modified();
 }
 
 void SubtitlesModel::doEditTrack(const SubtitlesModel::SubtitleTrack &track, int trackIndex)
@@ -562,6 +565,7 @@ void SubtitlesModel::doEditTrack(const SubtitlesModel::SubtitleTrack &track, int
     m_tracks[trackIndex] = track;
     emit dataChanged(index(trackIndex), index(trackIndex));
     emit tracksChanged(m_tracks.size());
+    emit modified();
 }
 
 void SubtitlesModel::doRemoveSubtitleItems(int trackIndex,

src/models/subtitlesmodel.h

@@ -99,6 +99,7 @@ class SubtitlesModel : public QAbstractItemModel
 
 signals:
     void tracksChanged(int count);
+    void modified();
 
 protected:
     // Implement QAbstractItemModel