Improve Axes selection in Qt figure options.

[anntzer]

1. Make sure Axes "labels" in the input dialog are unique
   (QInputDialog.getItem returns the selected string, but we'll get the
   wrong index if two axes share e.g. the same title), by including the
   axes id() if necessary; conversely, don't include the id() in the
   user-facing UI unless necessary.

   For example, previously, after

   fig, axs = plt.subplots(2)
   axs[0].set(xlabel="foo"); axs[1].set(xlabel="foo")
   

   the input dialog would not allow selecting the second axes.

2. If the Axes artist has a "label" attached to it (per
   Artist.set_label), use it in priority for the input dialog.
   Normally, axes labels are not used (they are used for other artists
   for the legend, for example). This change allows us to attach a
   "(colorbar)" label to colorbar axes, which makes it easier to select
   the right axes.

   After

    fig, axs = plt.subplots(2)
    axs[0].imshow([[0]]); fig.colorbar(axs[0].images[0])
    axs[1].imshow([[0]]); fig.colorbar(axs[1].images[0])
   

   previously the input dialog would have the entries

    <anonymous AxesSubplot (id: 0x...)>
    <anonymous AxesSubplot (id: 0x...)>
    <anonymous AxesSubplot (id: 0x...)>
    <anonymous AxesSubplot (id: 0x...)>
   

   but now it has the entries

    <anonymous AxesSubplot> (id: 0x...)
    <anonymous AxesSubplot> (id: 0x...)
    (colorbar) (id: 0x...)
    (colorbar) (id: 0x...)
   

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[jklymak]

Wow, who knew axes had labels. What are they used for (other than whatever you mean by "Axes selecton in Qt figures")?

[anntzer]

Probably nothing (well, they're artists and all artists have labels as that's defined in the base class).

What I meant by "Axes selection in Qt figure options" is