[FIX] heatmap: Ensure minimim size for color annotations

[ales-erjavec]

Issue

Ref: #4443 (comment)

Description of changes

Ensure minimum width on the color annotations.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@4fd0a48). Click here to learn what that means.
The diff coverage is 100.00%.

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[markotoplak]

@ales-erjavec, since #4443 I also get a test failure on Macs:

Traceback (most recent call last):
  File "/Users/marko/dev/orange3/Orange/widgets/utils/tests/test_graphicstextlist.py", line 55, in test_alignment
    self.assertEqual(brect(item).bottom(), 100)
AssertionError: 100.703125 != 100

[Hrovatin]

The annotation is now visible (although the numbers in the legend is marginally cut off).

[lanzagar]

Unless @ales-erjavec would like to try anything else regarding the right margin, I think we can merge this as is...