Cleaning up a few X scroll bar bits.

Cleaning up a few X scroll bar bits.
* termhooks.h (enum scroll_bar_part): Add scroll_bar_nowhere member.
* xterm.h (struct scroll_bar) [USE_TOOLKIT_SCROLL_BARS && USE_LUCID]:
New member last_seen_part, going to replace...
* xterm.c [USE_TOOLKIT_SCROLL_BARS]: ...global last_scroll_bar_part.
(xt_action_hook) [USE_LUCID]: Adjust user.
(xm_scroll_callback, xg_scroll_callback): Do not bloat with
Lucid-specific scroll bar support.
(xaw_jump_callback, xaw_scroll_callback): Prefer enum scroll_par_part
to int and adjust to use last_seen_part member.
(x_set_toolkit_scroll_bar_thumb) [USE_LUCID]: Adjust user.
(x_scroll_bar_create) [USE_TOOLKIT_SCROLL_BARS && USE_LUCID]:
Initialize last_seen_part.