plugin: cleanup boilerplate code & document

Now that MetaPlugin uses G_DECLARE_DERIVABLE_TYPE, we can get rid of some GObject boilerplate by using G_DECLARE_FINAL_TYPE.

Also follow a convention to put the _class_init and _init functions at the bottom, so we don't have to declare each vfunc override at the top.

Finally, I added some documentation at the top so people who have no idea about Mutter and its plugin system know how it works.