[rarmanager] do not show the process dialog unless warn size is exceeded

This commit re-uses the already defined warn size of 50MB to determine whether or not to show the process dialog when extracting compressed files from within a rar (eg. subs). Current behavior is to show the process dialog for files > 1 MB. In most cases the process dialog is just shown briefly as the actual extraction task takes less than opening and closing the dialog. This leads to a really bad user experience.