Add new option to merge all audio streams when using multi stream clip

Don't reload clip on audio channel change (fixed in MLT > 6.20)

src/bin/projectclip.cpp

@@ -963,7 +963,7 @@ void ProjectClip::setProperties(const QMap<QString, QString> &properties, bool r
     };
     QStringList forceReloadProperties{QStringLiteral("autorotate"),  QStringLiteral("templatetext"),   QStringLiteral("resource"),
                                       QStringLiteral("force_fps"),   QStringLiteral("set.test_image"), QStringLiteral("set.test_audio"),
-                                      QStringLiteral("audio_index"), QStringLiteral("video_index")};
+                                      QStringLiteral("video_index")};
     QStringList keys{QStringLiteral("luma_duration"), QStringLiteral("luma_file"), QStringLiteral("fade"),     QStringLiteral("ttl"),
                      QStringLiteral("softness"),      QStringLiteral("crop"),      QStringLiteral("animation")};
     QVector<int> updateRoles;

src/mltcontroller/clippropertiescontroller.cpp

@@ -618,7 +618,10 @@ ClipPropertiesController::ClipPropertiesController(ClipController *controller, Q
                 i.next();
                 audioStream->addItem(QString("%1: %2").arg(i.key()).arg(i.value()), i.key());
             }
-            if (!vix.isEmpty() && vix.toInt() > -1) {
+            if (audioStream->count() > 1) {
+                audioStream->addItem(i18n("Merge all streams"), INT_MAX);
+            }
+            if (!vix.isEmpty() && vix.toInt() != -1) {
                 audioStream->setCurrentIndex(audioStream->findData(QVariant(vix)));
             }
             ac->setActive(vix.toInt() == -1);