Bug 5177: Scroll bar thumb did not move when scrolling with mouse wheel.

Bug 5177: Scroll bar thumb did not move when scrolling with mouse wheel.

* xterm.c (xg_scroll_callback): Parameter list changed,
use parameter GtkScrollType to determine scroll/line/page.
Only allow dragging if a button < 4 is grabbed (bug #5177).
(xg_end_scroll_callback): New function.
(x_create_toolkit_scroll_bar): Pass xg_end_scroll_callback to
xg_create_scroll_bar.

* gtkutil.c (xg_gtk_scroll_destroy): Remove XG_LAST_SB_DATA handling.
(scroll_end_callback): Remove.
(xg_create_scroll_bar): Add parameter end_callback, bind it to
button-release-event.  Replace value-changed event with change-value,
bug #5177,
(xg_event_is_for_scrollbar): Only return true if button is less than 4,
bug #5177.

* gtkutil.h (XG_LAST_SB_DATA): Remove.
(xg_create_scroll_bar): Add GCallback end_callback.