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/LayoutItem.dox

@@ -6,8 +6,8 @@
  *		Alex Wilson, [email protected]
  *
  * Corresponds to:
- *		headers/os/interface/LayoutItem.h  rev 38207
- *		src/kits/interface/LayoutItem.cpp  rev 38207
+ *		headers/os/interface/LayoutItem.h  rev 49040
+ *		src/kits/interface/LayoutItem.cpp  rev 49040
  */
 
 
@@ -55,6 +55,21 @@
 */
 
 
+/*!
+	\fn bool BLayoutItem::RemoveSelf()
+	\brief Remove this layout item from the BLayout it is a part of.
+
+	This method works analoguous to BView::RemoveSelf(), that is it removes
+	itself from the parent layout and returns whether or not this was
+	succesful.
+
+	\return \c true if the item was removed from the layout, or \c false if
+		item was not part of a layout, or removal was succesful.
+
+	\since Haiku R1
+*/
+
+
 /*!
 	\fn BLayout::~BLayout()
 	\brief Destructor method.
@@ -188,6 +203,20 @@
 */
 
 
+/*!
+	\fn void BLayoutItem::SetExplicitSize(BSize size)
+	\brief Set the explicity size to \a size.
+
+	This method is a convenience method that sets the minimum size, maximum
+	size and preferred size to \a size, as to force this layout item to have
+	a specific size.
+
+	\param size The specific (non-zero) size for this layout item.
+
+	\since Haiku R1
+*/
+
+
 /*!
 	\fn void BLayoutItem::SetExplicitAlignment(BAlignment alignment) = 0
 	\brief Set this item's explicit alignment, to be used in Alignment().