company-pseudo-tooltip-frontend: Use one overlay per tooltip line to imp...

Currently, `company-pseudo-tooltip-frontend` uses one big overlay to cover the entire range of lines spanned by the tooltip. This disrupts the display of the region if other modes are also using overlays to convey visual information, such as linum, semantic-decoration, etc, creating an annoying visual distraction, especially when typing very quickly.

This patch reimplements company-pseudo-tooltip-frontend using additional overlays (one-per-tooltip-line) to more accurately cover the rectangular region occupied by the tooltip.

Known issues:
- Should probably limit maximum tooltip width,  to prevent overly long candidate items from messing up display.