[ENH] Export image background

[ales-erjavec]

Issue

Related to biolab/orange3#5680

If the background is not exported the displayed elements can have poor contrast depending on the style.
E.g. using biolab/orange3#5680 and a dark style:

Description of changes

Export image background, where not already done so.

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #181 (fb5083d) into master (3ad3888) will increase coverage by 2.13%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   68.87%   71.01%   +2.13%     
==========================================
  Files          35       34       -1     
  Lines        8739     8493     -246     
==========================================
+ Hits         6019     6031      +12     
+ Misses       2720     2462     -258     

Impacted Files	Coverage Δ
orangewidget/io.py	87.68% <79.16%> (-0.22%)	⬇️
orangewidget/workflow/discovery.py	62.96% <0.00%> (-3.71%)	⬇️
orangewidget/gui.py	47.25% <0.00%> (-0.06%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3ad3888...fb5083d. Read the comment docs.