Improving the single-instance IPC

Problem

Some file explorers open multiple folders with `process folder1 & process folder2` instead of `process folder1 folder2`

If "folder2" takes

between ,5 and 5 s to recurse it will remove the playlist entries added by "folder1"

longer than 5 s to recurse it will open "folder2" in a new instance

Solution

Improving the ,5 s limit by counting it from when the previous instance finished opening rather than started opening

Removing the 5 s 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