Improved CountDown timer

Lots of improvements over original timer. They can be divided into several
categories:

- Improve timer handler. The original timer counted seconds via a callback.
  If the computer was suspended, the timer would have been suspended too.
  The new timer continues to run even when the computer is suspended.

- Improved configurability of countdown timer
  Many of its properties can now be configured via object attributes

- Added menu bar:
  A menu bar item allows to start/pause/resume/cancel a timer

- Optional progress messages:
  It can optionally display messages to the screen as the timer is advancing

- Improved time-up messages.
  I found that the end of the timer notifications were too subtle to be noticed.
  It now allows several ways to configure the notifications

- A callback.
  User can specify a callback to the evaluated as the timer is
  started/paused/resumed/cancelled.

- In addition to minutes, a timer can now be set using a time of day,