Allow canceling a timer

[falkoschindler]

As requested in #1477, this PR adds the possibility to cancel a timer.
This stops the infinite while loop. Afterwards the timer cannot be activated again.

[bergmansj]

Looks good. Thanks for picking this up so quickly 👍

[rodja]

@falkoschindler I would like to use .remove() instead of .cancel() because it better carries the information of not being able to resume it.

[falkoschindler]

@rodja First I chose .stop(), but then went for .cancel() to indicate the destructive nature of this method. I'm not sure about .remove(), because it suggests a similarity to an element, which the timer isn't. But I'll think about it once more.

[rodja]

Ok. .remove() is to close to elements. I thought about .abort() but that is very similar to .cancel(). So we can go with it, I think.

[natankeddem]

I like.destroy() if you guys are taking suggestions. It seems fairly popular with some JS libraries.

[rodja]

Cool @natankeddem. I'm also in favour of .destroy() because it hints at the internal ._cleanup() method which releases the .slot and .callback members. The object is practically unusable after calling.

[falkoschindler]

I was about to push a commit renaming cancel() to destroy().

But the demo feels weird:
The title "Activate, deactivate and cancel a timer" becomes "Activate, deactivate and destroy a timer"
And the "Cancel" button becomes a "Destroy" button.

Other example: In a code-review, would you really say "Oh, we need to destroy the timer when the user clicks this button"? I think I would say "cancel" even though I would be writing "destroy". That's not a good sign.

[bergmansj]

In async frameworks tasks that are interrupted before finishing and in general cannot be resumed, the term 'cancelled' is used.

[rodja]

I'm now convinced .cancel() is the right name for it.