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 ",".

- The axes selector now uses shorter, more user-friendly names for axes, and

does not crash if there are no axes.

- Line and image entries using the default labels ("_lineX", "_imageX") are now

sorted numerically even when there are more than 10 entries.