Allow setting image clims in Qt options editor.

[anntzer]

In this implementation, (cmin, cmax) is swapped if cmin > cmax because it's not so easy to impose the constraint using formlayout...

[tacaswell]

👍 thanks! At somepoint can you write up a unified whats_new entry for all of the improvements you have been making to the config window?

[anntzer]

Something like the following?

Improvements for the Qt figure options editor
---------------------------------------------

Various usability improvements were implemented for the Qt figure options
editor, among which:
- Line style entries are now sorted without duplicates.
- The colormap and normalization limits can now be set for images.
- Line edits for floating values now display only as many digits as necessary
  to avoid precision loss.  An important bug was also fixed regarding input
  validation using Qt5 and a locale where the decimal separator is ",".
- Use a shorter, more user-friendly name in the axes selector, and do not crash
  if there are no axes.

If that looks good to you, can you PR it in?