Adds horizontal scrollbar to Fl_Tree as an ABI 1.3.3 feature.

***************************************************************
NOTE: You MUST uncomment the FLTK_ABI_VERSION in Enumerations.H
      to use these changes.
***************************************************************

Also: separated tree size calculation from draw() code,
so that one can cause the tree to recalculate immediately
after making modifications to the tree by calling Fl_Tree::calc_tree().

Numerous improvements to docs for the tree as well, enough
to create a rather large diff.

Large internal changes were needed to do this properly.
The following was added to the CHANGES file:

	- Fl_Tree: various related changes:
	    o Added horizontal scrollbar
	    o Separated draw() and tree size calculation
	    o Added new public methods:
	        > resize()           -- uses optimized dim calc, avoids tree recalc
		> next_item()        -- added parameters: direction, visibility
		> extend_selection() -- added parameters, improved algorithm
		> calc_dimensions()  -- calc tix/y/w/h, tox/y/w/h and scrollbars
		> calc_tree()        -- calc tree_w/tree_h
		> recalc_tree()      -- schedules calc_tree()
		> first_visible_item(), last_visible_item(), next_visible_item()
		> first_selected_item(), last_selected_item(), next_selected_item()
	    o Added protected variables:
	        > _tix/y/w/h      -- tree widget 'inner' dimension
		> _tox/y/w/h      -- tree widget 'outer' dimension
		> _tree_w,_tree_h -- entire tree hierarchy width/height
	    o Deprecated:
	        > item_clicked() -- use callback_item() instead
		> first_visible() -- use first_visible_item() instead
		> last_visible() -- use last_visible_item() instead

	- Fl_Tree_Item: various related changes:
	    o Added Fl_Tree ptr: needed for auto-recalc when item modified directly
	    o Added new methods tree(), recalc_tree()
	    o Added new ctor that accepts Fl_Tree*
	    o draw() parameters changed to include tree size calculations
	    o Deprecated:
	       > ctor using Fl_Tree_Prefs parameter (Fl_Tree* version better,
	         and must be used for 1.3.3 ABI features to work correctly)
	       > next_displayed() -- use next_visible() instead
	       > prev_displayed() -- use prev_visible() instead

	- test/tree: added tests for newly added features




git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10034 ea41ed52-d2ee-0310-a9c1-e6b18d33e121