Clean up include macros mess

The order and way include macros were structured was chaotic, with no
real common thread between files. Try to tidy up the mess with some
common scheme, to make things look less messy.

src/core/stack.h

@@ -40,7 +40,7 @@
  * by layer, then by stack_position within each layer.
  */
 
-#include "display-private.h"
+#include "core/display-private.h"
 
 /**
  * A sorted list of windows bearing some level of resemblance to the stack of