WIP: Adjust nticks based on length of axis

[mdboom]

There's a bit of a chicken and egg problem here. You don't know how many ticks you can fit if you don't know what the tick labels contain. And you don't know what the tick labels contain until you know how many ticks there are.

Therefore this is just based on a heuristic that the text will have an aspect ratio of around 4:1. Probably not always true, and there are perhaps better ways to estimate based on what the tick formatter actually is, or by rendering just one of them and using that.

[Tillsten]

Very nice to see some work on this, adjusting ticks always takes quite a lot of time. Feature request: ability to disallow ticks near the end of an axis, this is very helpful when stacking axes without any hspace/wspace, because else the ticklabels are overlapping.

[mdboom]

I'm going to close this and move it to the style-change branch. It's not possible to make this backward compatible without some other changes over there.