mplay32: don't use the timer proc to check if playback should stop

Remove the code that checks if the currently playing media has reached its end from PlayTimerProc,
so that is only contains code that updates the user interface. Instead, handle MM_MCINOTIFY
messages and stop / start again the media if a success notification is recived.
Fixes an issue where the playback does not stop / repeat with some devices if it has reached its end.