elementfactory: add DECRYPTOR class defines

elementfactory: add DECRYPTOR class defines

An element that performs decryption does not naturally fit within any
of the existing element factory class types. It is useful to be able
to easily get a list of all elements that support decryption so that
a union can be computed between the protection systems that have a
supported decryptor and the allowed protection systems for a particular
stream.

This commit adds a new GST_ELEMENT_FACTORY_TYPE_DECRYPTOR and its
associated string identifier "Decryptor". It also adds
GST_ELEMENT_FACTORY_TYPE_DECRYPTOR to GST_ELEMENT_FACTORY_TYPE_DECODABLE
so that uridecodebin can auto-plug a decryption element.

https://bugzilla.gnome.org/show_bug.cgi?id=705991