SUBDIRECTORIES_RECURSE: add new directory property

This commit adds a new directory property SUBDIRECTORIES_RECURSE, which collects all subdirectories recursively, so that later on in cmake code you can iterate over all the subdirs and get their directory properties and do something, e.g. get all targets in the directories etc. This way users don't have to implement collecting the subdirectories recursively in cmake code.

This topic was originally raised in the forums here: https://discourse.cmake.org/t/adding-a-subdirectories-recurse-directory-property

Topic-rename: prop-SUBDIRECTORIES_RECURSE