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/GridLayout.dox

@@ -107,7 +107,7 @@
 
 
 /*!
-	\fn void BGridLayout::SetHorizontalSpacing(float spacing);
+	\fn void BGridLayout::SetHorizontalSpacing(float spacing)
 	\brief Set the spacing between columns for this layout.
 
 	\param spacing The number of pixels of spacing to set.
@@ -257,6 +257,20 @@
 */
 
 
+/*!
+	\fn BLayoutItem* BGridLayout::ItemAt(int32 column, int32 row) const
+	\brief Get a pointer to the item at the requested cell in the grid.
+
+	\param column The column, starting with \c 0 for the leftmost column. Note
+		that the value should be 0 or higher.
+	\param row The row, starting with \c 0 for the topmost row. Note that the
+		value should be 0 or higher.
+
+	\return A pointer to the layout item at these coordinates, or \c NULL if
+		there is no item.
+*/
+
+
 /*!
 	\fn BLayoutItem* BGridLayout::AddView(BView* child)
 	\brief Adds \a child to this layout in the first empty cell available, or
@@ -315,3 +329,11 @@
 
 	\since Haiku R1
 */
+
+
+/*!
+	\fn static BArchivable* BGridLayout::Instantiate(BMessage *from)
+	\brief Instantiate the layout from the message \a from.
+
+	\since Haiku R1
+*/