Add summary-face keyword argument to popup-create-line-string
Add summary-face keyword argument to popup-set-line-item
Handle summary-face in popup-draw
Define popup-summary-face
Define popup-menu-summary-face
Do not use summary-face for selected item

The best way to do this is to dynamically create a
face based on selection-face and summary-face.
But this patch is still better than the older version
without summary-face.
Use face inheritance to avoid duplication
Merge pull request #30 from tkf/summary-face

Summary face
Merge pull request #31 from tkf/inherit-face

Inherit face