made auto-deleting cache on close an option

Previously we had no disk management whatsoever, so we cleared the cache on
every close just to prevent clogging up tester disk space. Now that we are
implementing disk management, there are better things to do on close regarding
disk cache. However, some users may still wish for the app to delete the cache
on close, so it's provided as an option.

app/core.cpp

@@ -43,6 +43,7 @@
 #include "project/item/footage/footage.h"
 #include "project/item/sequence/sequence.h"
 #include "render/colormanager.h"
+#include "render/diskmanager.h"
 #include "task/import/import.h"
 #include "task/taskmanager.h"
 #include "ui/style/style.h"
@@ -129,6 +130,8 @@ void Core::Stop()
 
   AudioManager::DestroyInstance();
 
+  DiskManager::DestroyInstance();
+
   NodeFactory::Destroy();
 
   delete main_window_;
@@ -376,6 +379,9 @@ void Core::StartGUI(bool full_screen)
   // Initialize audio service
   AudioManager::CreateInstance();
 
+  // Initialize disk service
+  DiskManager::CreateInstance();
+
   // Connect the PanelFocusManager to the application's focus change signal
   connect(qApp,
           SIGNAL(focusChanged(QWidget*, QWidget*)),