qtdemux: add support for ISOBMFF Common Encryption
This commit adds support for ISOBMFF Common Encryption (cenc), as defined in ISO/IEC 23001-7. It uses a GstProtection event to pass the contents of PSSH boxes to downstream decryptor elements and attached GstProtectionMeta to each sample. https://bugzilla.gnome.org/show_bug.cgi?id=705991