fix: panel/window bugs surfaced by new gtest coverage

- MainWindowLayoutInfo: toXml() iterated open_sequences_ for the
  <viewers> section and fromXml() never parsed it, so open footage
  viewers were lost on layout save/load
- PanelWidget never set QObject::objectName, so
  PanelManager::GetPanelWithName() always returned nullptr and every
  caller (layout restore, NodeView param panel lookup) was dead code
- PanelManager::DestroyInstance() left instance_ dangling (UAF on any
  later RegisterPanel), unlike the other singletons

app/window/mainwindow/mainwindowlayoutinfo.cpp

@@ -48,10 +48,10 @@ void MainWindowLayoutInfo::toXml(QXmlStreamWriter *writer) const
 
 	writer->writeStartElement(QStringLiteral("viewers"));
 
-	foreach (Sequence *sequence, open_sequences_) {
+	foreach (ViewerOutput *viewer, open_viewers_) {
 		writer->writeTextElement(
 			QStringLiteral("viewer"),
-			QString::number(reinterpret_cast<quintptr>(sequence)));
+			QString::number(reinterpret_cast<quintptr>(viewer)));
 	}
 
 	writer->writeEndElement(); // viewers
@@ -129,6 +129,19 @@ MainWindowLayoutInfo::fromXml(QXmlStreamReader *reader,
 				}
 			}
 
+		} else if (reader->name() == QStringLiteral("viewers")) {
+			while (XMLReadNextStartElement(reader)) {
+				if (reader->name() == QStringLiteral("viewer")) {
+					quintptr item_id = reader->readElementText().toULongLong();
+
+					ViewerOutput *open_viewer =
+						static_cast<ViewerOutput *>(node_ptrs.value(item_id));
+					info.open_viewers_.push_back(open_viewer);
+				} else {
+					reader->skipCurrentElement();
+				}
+			}
+
 		} else if (reader->name() == QStringLiteral("state")) {
 			info.state_ =
 				QByteArray::fromBase64(reader->readElementText().toLatin1());