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/panel/panel.cpp

@@ -42,6 +42,11 @@ PanelWidget::PanelWidget(const QString &object_name)
 	, border_visible_(false)
 	, signal_instead_of_close_(false)
 {
+	// KDDockWidgets stores object_name as its uniqueName but does not set it
+	// as the QObject::objectName; PanelManager::GetPanelWithName() (and layout
+	// restore in MainWindow) look panels up by objectName, so mirror it here
+	setObjectName(object_name);
+
 	View<QWidget>::setFocusPolicy(Qt::ClickFocus);
 
 	connect(this, &PanelWidget::shown, this,