renderer: Turn PlayerController into an interface

There is a need to be able to reimplement the rygel-player-controller
By changing the class to become an interface, other classes can
implement it.
The current implementation is moved to a default controller.
This controller is instantiated by get_controller of the plugin as
before, but now the get_controller function is defined virtual.
Making it possible for plugins to reimplement and return another
controller.

Signed-off-by: Richard Röjfors <richard.rojfors@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=732057
all: Add gobject-introspection support

Signed-off-by: Jens Georg <mail@jensge.org>
examples: Add python examples for introspection

Signed-off-by: Jens Georg <mail@jensge.org>
build: Fix distcheck

This is probably wrong, it should not rebuild this in the first place

Signed-off-by: Jens Georg <mail@jensge.org>