Minimize the unpleasent visual impact of the requirement

Minimize the unpleasent visual impact of the requirement
that non-toolkit tool-bars must occupy an integral number of screen
lines, by distributing the rows evenly over the tool-bar screen	area.
(Vtool_bar_border): New variable.
(syms_of_xdisp): DEFVAR_LISP it.
(display_tool_bar_line): Add HEIGHT arg for desired row height.  Make
tool-bar row the desired height.  Use default face for border below
tool-bar.
(tool_bar_lines_needed): Add N_ROWS arg.  Use it to return number of
actual tool-bar rows.
(redisplay_tool_bar): Calculate f->n_tool_bar_rows initially.
Adjust the height of the tool-bar rows to fill tool-bar screen area.
(redisplay_tool_bar): Calculate f->n_tool_bar_rows when tool-bar area
is resized.