Add tickson: 'boundaries' for category cartesian axes

[etpinard]

Closes#2601 - adding tickson to date axes and special spike support should get issues of their own.

cc @plotly/plotly_js

[alexcjohnson]

Do we want a longer default ticklen when you turn on tickson: 'boundaries'? And we definitely need to not push the labels away from the axis (ie to be beyond the ticks).

There are probably also some unpleasant effects on collision avoidance. Imagine the case:
'A very long title', 'short', 'Another very long title'
They could all avoid touching each other yet overlap the boundaries which would still be unacceptable, at least if we're showing ticks, though it might still be better to rotate in this case even with just grid lines. And perhaps we want to only auto-rotate to 90 degrees, omitting the step at 30 degrees?

[etpinard]

Thanks very much @alexcjohnson for the thorough review. I overlooked many subtle details. 🤕

And we definitely need to not push the labels away from the axis (ie to be beyond the ticks).

Oh right. Done in -> 78ff4d5

There are probably also some unpleasant effects on collision avoidance. Imagine the case:
'A very long title', 'short', 'Another very long title'
They could all avoid touching each other yet overlap the boundaries which would still be unacceptable, at least if we're showing ticks, though it might still be better to rotate in this case even with just grid lines. And perhaps we want to only auto-rotate to 90 degrees, omitting the step at 30 degrees?

I went with auto-rotating straight to 90 degrees with or w/o ticks in -> 09ac230

Do we want a longer default ticklen when you turn on tickson: 'boundaries'?

I'm not opposed. How much longer are you thinking?

[alexcjohnson]

Do we want a longer default ticklen when you turn on tickson: 'boundaries'?

I'm not opposed. How much longer are you thinking?

Hmm, looking at it in more detail, I think perhaps the current default is fine. Definitely once we get to multi-level axes, ticks for the outer levels will need to by default extend beyond the inner level's labels, but we can leave it as is for now.

[alexcjohnson]

nice solution, keeping xbnd with the rest of the tick calculations.

[alexcjohnson]

Nice, just squeaks in unrotated if we draw this without tickson: 'boundaries' 🎉

[alexcjohnson]

OK, looks great! 💃