Formatter for plot axis labels

[Bromeon]

Adds two methods Plot::x_axis_formatter and Plot::y_axis_formatter().
Also fixes the bug where Orientation was used in public fields, but wasn't public itself.

With them, it's possible to customize the displayed labels along the axes.

Plot::new("Complex plane")
    .y_axis_formatter(|val| format!("{}i"))
    .show(ui, |plot_ui| plot_ui.bar_chart(chart))

It's also possible to return the empty string, in order to not display a label. This can be useful for discrete X/Y domains, when there isn't a meaningful label beyond certain resolution or zoom level. An example can be dates, with only full days.

Here's a screenshot of the stacked bar-chart, with X axis labeled as days and Y axis as percentage.
This example is now implemented in the plot demo.

Closes #725.

[Bromeon]

One open question is positioning of the labels.

In the screenshot above I had to "hide" the Y=0 value 0%, because it would overlap with the X=0 label Day 0. I did this by returning an empty string in the formatter.

Problems like these could be solved by specifying the alignment (left/right/center) and orientation (horizontal/vertical) of the text along the axis. But I think it's an orthogonal problem to text formatting (no pun intended), so it should be possible to do it in a separate PR.

[Bromeon]

Fixed merge conflicts.

[emilk]

Looks good! Just left a couple of nits

[Bromeon]

Applied the suggestions.

---

Sorry for the many CI runs, wasn't aware that previous runs would complete. In case you're interested, it's possible to abort previous runs as done here:

# If a new commit is pushed before the old one's CI has completed (on the same branch), abort previous run
concurrency:
  group: ${{ github.head_ref }}
  cancel-in-progress: true

This might only work for the pull_request and not the push trigger however (not sure if you use CI for the latter, too, or if you do all changes via PRs).