[RFC][ENH] OWWidget: Input/output summary

[ales-erjavec]

Issue

Provide a consistent widget input/output descriptions to the user.

Description of changes

Works on top of #2514

Add an input and output message areas in the widget's status bar.
The widget authors set the summaries using

self.info.set_input_summary("foo")
self.info.set_output_summary("bar")

for basic inline short descriptions. A more detailed (including rich text) summaries are also possible:

Ignore stylistic deficiencies in the last commit. Its flailing around to find some common 'description language'.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2556 into master will increase coverage by 0.05%.
The diff coverage is 96.76%.

@@            Coverage Diff             @@
##           master    #2556      +/-   ##
==========================================
+ Coverage   83.17%   83.23%   +0.05%     
==========================================
  Files         362      362              
  Lines       63832    64088     +256     
==========================================
+ Hits        53094    53344     +250     
- Misses      10738    10744       +6

[astaric]

@markotoplak, @BlazZupan, @janezd, this has been sitting here for too long.

If rebased, could it be included in the next release?

[markotoplak]

This line will probably have to updated. :)

[ajdapretnar]

This didn't make it into the 3.15.0 release, so where are we at?

[ajdapretnar]

@BlazZupan Can we merge this before 3.16 or at least decide what to do with it? @lanzagar

[BlazZupan]

This looks really cool, @ales-erjavec can you please resolve conflicts and then we merge.

[janezd]

respectively

[BlazZupan]

Once merged, this functionality would also require a note in a widget development documentation. @ales-erjavec, can you please add an entry there as well.