Make MetaOutput a GObject

Turn MetaOutput into a GObject and move it to a separate file. This
changes the storage format, resulting in changing the API for accessing
MetaOutputs from using an array, to using a GList.

https://bugzilla.gnome.org/show_bug.cgi?id=785381

src/backends/native/meta-cursor-renderer-native.c

@@ -38,6 +38,7 @@
 #include "backends/meta-logical-monitor.h"
 #include "backends/meta-monitor.h"
 #include "backends/meta-monitor-manager-private.h"
+#include "backends/meta-output.h"
 #include "backends/native/meta-renderer-native.h"
 #include "core/boxes-private.h"
 #include "meta/boxes.h"