Initial pass on API stability guarantees

This introduces -DWLR_USE_UNSTABLE and adds information regarding the
stability status to all headers. I started with a conservative set of
headers to mark as stable:

- types/wlr_matrix.h
- util/edges.h
- util/log.h
- util/region.h
- xcursor.h

include/wlr/backend/multi.h

@@ -1,3 +1,11 @@
+/*
+ * This an unstable interface of wlroots. No guarantees are made regarding the
+ * future consistency of this API.
+ */
+#ifndef WLR_USE_UNSTABLE
+#error "Add -DWLR_USE_UNSTABLE to enable unstable wlroots features"
+#endif
+
 #ifndef WLR_BACKEND_MULTI_H
 #define WLR_BACKEND_MULTI_H