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/config/config.cpp

@@ -76,6 +76,7 @@ void Config::SetDefaults()
 
   config_map_["DiskCachePath"] = QStandardPaths::writableLocation(QStandardPaths::AppLocalDataLocation);
   config_map_["DiskCacheSize"] = 20.0;
+  config_map_["ClearDiskCacheOnClose"] = false;
 
   config_map_["DefaultSequenceWidth"] = 1920;
   config_map_["DefaultSequenceHeight"] = 1080;