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/xcursor.h

@@ -24,8 +24,19 @@
  */
 
 /*
- * This is adapted from wayland-cursor, but with the wl_shm client stuff removed
- * so we can use it on the compositor, too.
+ * This is a stable interface of wlroots. Future changes will be limited to:
+ *
+ * - New functions
+ * - New struct members
+ * - New enum members
+ *
+ * Note that wlroots does not make an ABI compatibility promise - in the future,
+ * the layout and size of structs used by wlroots may change, requiring code
+ * depending on this header to be recompiled (but not edited).
+ *
+ * Breaking changes are announced by email and follow a 1-year deprecation
+ * schedule. Send an email to ~sircmpwn/wlroots-announce+subscribe@lists.sr.ht
+ * to receive these announcements.
  */
 
 #ifndef WLR_XCURSOR_H