The notification had a top margin of 8px but no corresponding bottom
marging, making notifications look unbalanced. Added matching bottom
margin.

Also increased the image padding to make it horizontally symetrical and
removed several unused defines.

Fixes #116

When compositing is disabled, notifications have a rounded shape mask
applied but it's not redrawn. This change queues a redraw after applying
the shape region so the notification shows rounded corners.

Fixes #98

Countdown was only visible for notifications that had action buttons.
This change adds a 'show-countdown' gsetting to display it instead on
all non-persisntent notifications.

Allow to set coutdown color from .countdown:selected for all themes
Fix the countdown for slider theme and draw transparent background for countdown