New variable company-tooltip-maximum-constant-width

This provides better control over the tooltip width when not all candidates can
be shown (eg, to avoid a jagging motion when scrolling through candidates).

The default value 0 shows (as before) a tooltip wide enough for the currently
visible candidates' lines.  A positive integer will reserve up to this many
characters for any candidate (even those not shown), but never more than what
all current candidates need.  To always reserve enough horizontal space to
accommodate all current candidates, set to a large integer.