Support for adding ad-hoc exercises, that don't have a specific interface, just a timer and some description.

[kwyckmans]

There's a bunch of exercises that have no specific interface I want to use. Just a sort of reminder that I need to do them. They need a name, a description, maybe some notes and a timer.

[kwyckmans]

I have a component for generic exercises that has a name, a description and a timer. I have no way of creating new exercises yet, nor do I have a way of persisting them.

[kwyckmans]

A bug that I maybe need to fix: timer is shared between both components. I have 2 "generic" exercises, if I start the timer on the first one, it is shared with the 2nd one. I would expect them to be either:

• Individual: I can start multiple concurrent timers.
• Reset: If I switch from one exercise to another, the timer resets, since I don't want to do multiple concurrent exercises.

See #4