Fix tooltip positioning when displayed above point.

* company.el

(company-pseudo-tooltip-show): Refactored arguments, added
documentation.

(company-pseudo-tooltip-show-at-point): Adjust height based on
actual number of lines in tooltip.
Avoid saving padded newlines to disk.

* company.el:

(company--buffer-save-commands): New constant.

(company-pseudo-tooltip-frontend): If this-command can potentially
save current buffer to disk, then completely hide tooltip before
running command, then redraw tooltip on post-command.