Add public API for WCSAxes to retrieve the tick locations and labels on each axis

I am interested in being able to plot world coordinates in non-Matplotlib viewers, for example in bqplot. However, I would prefer to not have to re-implement all of WCSAxes in bqplot, and an alternative would be to internally store a WCSAxes instance and set limits on it, then query it for tick locations and labels. However, I don't think we have a proper way at the moment to get this - and:

```python
ax.xaxis.get_ticklocs()
ax.yaxis.get_ticklabels()
```

is not correct because it is what the ticks would have been in the absence of WCSAxes. We might want to consider having e.g. ``CoordinateHelper.get_ticks()`` and ``CoordinateHelper.get_ticklabels()`` for this.

Start by reviewing WCSAxes and CoordinateHelper, including how ax.xaxis and ax.yaxis currently expose tick information. Define the public API needed to return WCS-aware tick locations and labels for each coordinate axis, with behavior suitable for non-Matplotlib viewers such as bqplot.

Written by the indexing model from the issue text.