mediafile: Turn into an extension point

mediafile: Turn into an extension point

This way, we can support external libraries providing implementations of
GtkMediaFile.

We also add a media backend called 'nomedia' that can be enabled to not
compile any support for GtkMediaFile. This is useful when people want to
statically compile GTK into an application that does not use media.
For now, this option is the default.

We also support a new environment variable GTK_MEDIA that allows
selecting the implementation to use.
GTK_MEDIA=help can be used to get info about the available
implementations.