target argument of MovieClipLoader#loadClip and unloadClip may be level

According to the document[1,2], "target" argument of these methods may
be number (level).

1: http://help.adobe.com/en_US/AS2LCR/Flash_10.0/help.html?content=00001381.html
2: http://help.adobe.com/en_US/AS2LCR/Flash_10.0/help.html?content=00001389.html