moved PanelManager to a more traditional singleton form

app/panel/panelmanager.cpp

@@ -22,7 +22,7 @@
 
 #include "config/config.h"
 
-PanelManager* olive::panel_manager = nullptr;
+PanelManager* PanelManager::instance_ = nullptr;
 
 PanelManager::PanelManager(QObject *parent) :
   QObject(parent),
@@ -78,6 +78,21 @@ bool PanelManager::ArePanelsLocked()
   return locked_;
 }
 
+void PanelManager::CreateInstance()
+{
+  instance_ = new PanelManager();
+}
+
+void PanelManager::DestroyInstance()
+{
+  delete instance_;
+}
+
+PanelManager *PanelManager::instance()
+{
+  return instance_;
+}
+
 void PanelManager::FocusChanged(QWidget *old, QWidget *now)
 {
   Q_UNUSED(old)