Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

The old struct config_param remains only for top-level string options.

Supports "~/" expansion.  Forces us to switch from "const char *" to
AllocatedPath, which is a good thing.

Make it "const".

.. and move the plugins to src/encoder/plugins/.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.