HaikuBook: add remaining layout class documentation

This is the final contribution to #15368

* Tried to share more documentation in the various BLayoutBuilder classes
* Add missing GridView, GroupView, SpaceLayoutItem
* Also added AbstractLayoutItem, but hide the actual documentation behind
  an `INTERNAL` conditional block. This block identifier can be used to
  document parts of the API, to then hide them during a regular Doxygen run.
* Do some cleanup on other layout classes; add missing members, etc.
* The actual generated BLayoutBuilder::* html is a mess. I should investigate
  this at a later time. Especially the copied members seem to mix type
  definitions with member documentation. It is odd. Not unlikely to be a
  Doxygen bug.
* The general documentation for the layout system could use an overhaul as
  well, but this is for later.

Change-Id: I6db9ef105b4ae6de0f1ebb917f86f8b1c0d4ea2e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2491
Reviewed-by: waddlesplash <[email protected]>

docs/user/interface/LayoutBuilder.dox

@@ -1,13 +1,12 @@
 /*
- * Copyright 2010 Haiku, Inc. All rights reserved.
+ * Copyright 2010, 2020 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  *
  * Authors:
  *		Alex Wilson, [email protected]
  *
  * Corresponds to:
- *		headers/os/interface/LayoutBuilder.h	rev 38207
- *		src/kits/interface/LayoutBuilder.cpp	rev 38207
+ *		headers/os/interface/LayoutBuilder.h	 rev 49977
  */
 
 
@@ -122,3 +121,15 @@ BLayoutBuilder::Group<>(B_HORIZONTAL)
 
 	\since Haiku R1
 */
+
+
+//! \cond INTERNAL
+
+
+/*!
+	\var ParentBuilder* BLayoutBuilder::Base< ParentBuilder >::fParent
+	\brief Internal pointer to the parent of the builder.
+*/
+
+
+//! \endcond