mp4sys: Refactor around construction and descruction.
mp4a: Fix a memory leak when setting up audio summary.

This is a regression at 21e2c63 .
mp4sys: Fix a memory leak when setting up audio summary.
mp4a: Set audioProfileLevelIndication properly.
mp4sys: Reduce slightly codes to set up audio summary.
mp4sys: Simplify update of descriptor size by using a fake bytestream writer.
mp4sys: Fix the size of the descriptor when the header length is variable.
mp4sys: Remove unused mp4sys_duplicate_ES_Descriptor().

Also remove related functions.
mp4sys: Use mp4sys_add_ES_ID_Inc() in mp4sys_get_ES_ID_Inc().
Implementation of referencing external media files.