(re)allow modifying ticks in afterBuildTicks

[kurkle]

A workaround proposal to regain lost functionality.
The 'internal' tick objects are exposed, but I only see it as a bonus for custom axis types.

Closes #4749

https://codepen.io/kurkle/pen/EGKWgM?editors=0010

[simonbrunel]

I'm not sure why I didn't think about that solution when refactoring this part of the scales, maybe because I was planning to deprecate all those callbacks. Anyway, looks like a good solution for now :)

To get the same results as #4749 (before 2.6.0), the user also needs to set scale.ticks.source: 'data' because the previous createFilterTicks functor directly uses the data as input (and not the generated ticks) (codepen) (in fact, we don't even need to filter the 'day' ticks in that case).

The 'internal' tick objects are exposed, but I only see it as a bonus for custom axis types.

It's already public via scale.getTicks().

@kurkle can you add a unit test?

[benmccann]

@kurkle this PR will need to be rebased as well