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/panelmanager.cpp

@@ -98,6 +98,7 @@ void PanelManager::CreateInstance()
 void PanelManager::DestroyInstance()
 {
 	delete instance_;
+	instance_ = nullptr;
 }
 
 PanelManager *PanelManager::instance()