Improvements for the Qt figure options editor
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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.