(#includes): Allow compilation with only Xaw.

(#includes): Allow compilation with only Xaw.
(xaw3d_arrow_scroll, xaw3d_pick_top): New variables.
(xt_action_hook): Replace XAW3D by XAW.
(xaw3d_jump_callback): Renamed to xaw_jump_callback.
(xaw_jump_callback): Renamed from xaw3d_jump_callback.
Determine epsilon dynamically and don't try to be too clever.
(xaw3d_scroll_callback): Renamed to xaw_scroll_callback.
(xaw_scroll_callback): Renamed from xaw3d_scroll_callback.
Handle both Xaw3d with arrow-scrollbars and with Xaw-style
scrollbar (using `ratio').
(x_create_toolkit_scroll_bar): Try to detect which style of Xaw3d
scrollbar we have so as to set it up more optimally and to fix
xaw3d_arrow_scroll and xaw3d_pick_top.
(x_set_toolkit_scroll_bar_thumb): Try to maintain 2 spare pixels at the
bottom of the Xaw3d scrollbar, to work around its tendency to refuse
shrinking the thumb.  Also make sure that `XawScrollbarSetThumb'
is not ignored, using a major gross hack.
(x_initialize): Init default values for xaw3d_arrow_scroll and
xaw3d_pick_top.