Improving the single-instance IPC

Changing the single-instance IPC so that it waits for the new process to perform a slow task (f.e. recursing through a large folder tree or multiple folders) rather than resetting the playlist after a time

OnCopyData was previously limited to 500 ms but now has no limit

The ID_FILE_CLOSEMEDIA is removed because it causes a long delay between WM_COPYDATA messages, causing the playlist to reset. If it has to be there the allowed delay between WM_COPYDATA messages has to be increased to at least 2000 ms