Reduce GTK tool-bar switching delay by avoiding selective show/hide of widgets.

Reduce GTK tool-bar switching delay by avoiding selective show/hide of widgets.

* src/gtkutil.c (xg_get_tool_bar_widgets): Use NULL for a missing
image or label in the container.
(xg_make_tool_item): Replace VERT_ONLY arg with HORIZ, TEXT_IMAGE.
(xg_show_toolbar_item): Function deleted.
(xg_tool_item_stale_p): New function.
(update_frame_tool_bar): Calculate tool-bar style once per call.
Instead of hiding text labels, omit them.  Don't use
xg_show_toolbar_item; create new GtkToolItems from scratch if
necessary, instead of trying to re-use them.  This avoids an
annoying animation when changing tool-bars.