Add a directory property to list subdirectories

Add a SUBDIRECTORIES directory property to allow project code to
traverse the directory structure of itself as CMake sees it.

Help/release/dev/directory-list-targets-and-subdirs.rst

@@ -0,0 +1,5 @@
+directory-list-targets-and-subdirs
+----------------------------------
+
+* A :prop_dir:`SUBDIRECTORIES` directory property was added to
+  get the list of subdirectories added by a project in a directory.