Add countdown timer and pomodoro timer

[Naerriel]

You set: working on aSkill for 30 minutes and then there is a countdown with a sound signal at the end.
It will be further developed into a pomodoro.

Btw. personally I would use all three possible timers depending on mood:
Normal timer - when you are in a flow and want to work for a long time and you don't know how long
Countdown timer - when you want to finish this one particular work session
Pomodoro timer - when you want to plan your breaks as well

Work system may vary from person to person and including all those 3 timers can satisfy their needs. Pomodoro may be the best in use but in order to implement pomodoro you have to implement the other 2 as well and I see no problem with that.