Improve tray icon notification behavior

Improve tray icon notification behavior

The tray icon now blinks as long as there are still buffers with unread highlights. As
added bonus, repeated clicks on the tray icon now switch to the next highlighted buffer in
line (rather than hiding the window).

Also, notifications (in KDE) will be properly closed as soon as you switch to the appropriate
buffer.

After all highlighted buffers have been visited, the icon stops blinking. Fixes #720.