[ENH] Scatter Plot: Error Bars

[VesnaT]

Issue

Add Error Bars to points.

Closes #6428.

Description of changes

• introduce a dialog for variable selection
• add error bars to plot

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Attention: Patch coverage is 97.38220% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.48%. Comparing base (c2c1648) to head (92837cf).
Report is 95 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6934      +/-   ##
==========================================
+ Coverage   88.45%   88.48%   +0.02%     
==========================================
  Files         331      332       +1     
  Lines       73099    73277     +178     
==========================================
+ Hits        64659    64838     +179     
+ Misses       8440     8439       -1     

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[janezd]

I had to change glasses to see what's actually in the button.

If any, I think it should be delta, not epsilon. But what I had in mind would be an icon like this.

It takes a bit more space (which I'm sorry for), but it's better to take space that to look weird. :)

I'd also slightly modify the dialog, like this:

I think it was not very clear what "Difference" and "Absolute" meant. Now it may be a bit better, though a tooltip would help.

One minor comment: "Upper" and "Lower" make more sense for y. For x, the order is also weird -- there I'd expect to set the lower limit first.

I cannot push a new commit to this PR, so I'm attaching a diff and the icons.

interval-horizontal
interval-vertical
addicons.patch