Draft: media: Don't build media modules as modules

This is fallout from a discussion with @xclaesse on IRC today.

It solves the problem of running uninstalled - both manually as well as part of the testsuite - as the media modules are built-in and no proper path into the modules build dir is needed.

This is somewhat brittle with the build system as the media modules should really be built as part of libgtk_static but that would require them being in a subdir of gtk/.