Image options

[anntzer]

This patch allows the user to interactively set an image's colormap from the "axes and lines (and now images) parameters editor" (in the Qt backend).

Colormaps are listed from those registered with matplotlib.cm. Colorbars seem to be handled properly.

Note: this patch builds onto my previous PR, #5342 (also involving the options editor). If #5342 gets rejected I will split out the specifics of this PR.

[tacaswell]

👍 This now needs a re-base due to merging #5342

Only suggestion would be to filter out the *_r colormaps and add a 'reverse' check box.

[anntzer]

Is there a generic way to get the reverse of a colormap? The current colormap could be a user-defined one, in which case it needs to be available in the combobox, but then the user may pick to use its reversed form instead, in which case I need to generate that colormap on the fly.
It does seem to be a good idea though.

[tacaswell]

There was some discussion of adding such a method, but I don't think it went anywhere. In cm.py there are functions that are used to reverse the specifications (and are how we generate the *_r color maps, but I do not know if they can be made to work on a generic color map.

If the checkbox is going to take more than 15min, I would say skip it for this PR.

[anntzer]

I'd say it will :-) (specifically for this part, the rest is not that hard)

[anntzer]

Kindly bumping the issue. (If there's no easy way to reverse a cmap I'll skip that functionality for now.)

[mdboom]

Seems fine, but this needs a rebase on current master.

[anntzer]

Done (with no rebase conflict).