macOS/QuickLook: support rich thumbnail in Finder

Support thumbnail that shows the file contents instead
of the default blend file icon for all files in Finder.
Some files may still have the usual blender icon as thumbnail
depending on their contents.

blender-thumbnailer process is kept alive by the system
in the background and is invoked by QuickLook when needed.

The checkbox to disable the extension is present in
the Settings app.

It will NOT be an interactive "Preview" that allows richer
interactions like Panning viewport, or rotating 3D objects.

CMakeLists.txt

@@ -216,20 +216,18 @@ Build \"BlendThumb.dll\" helper for Windows explorer integration to support extr
 thumbnails from `.blend` files."
     ON
   )
-else()
-  set(_option_default ON)
-  if(APPLE)
-    # In future, can be used with `quicklookthumbnailing/qlthumbnailreply`
-    # to create file thumbnails for say Finder.
-    # Turn it off for now, even though it can build on APPLE, it's not likely to be useful.
-    set(_option_default OFF)
-  endif()
+elseif(UNIX AND NOT APPLE)
   option(WITH_BLENDER_THUMBNAILER "\
 Build stand-alone \"blender-thumbnailer\" command-line thumbnail extraction utility, \
 intended for use by file-managers to extract PNG images from `.blend` files."
-    ${_option_default}
+    ON
+  )
+elseif(APPLE)
+  option(WITH_BLENDER_THUMBNAILER "\
+Build \"blender-thumbnailer.appex\" extension for macOS Finder/ QuickLook to support viewing \
+thumbnails from `.blend` files."
+    ON
   )
-  unset(_option_default)
 endif()
 
 option(WITH_INTERNATIONAL "Enable I18N (International fonts and text)" ON)