Color contrast survey updates

[tombrunet]

Changes include:

• Removes the restriction of applicability to widgets. Maintains exclusion for disabled widgets.
  • This caused inapplicable examples to now passed using default browser colors
  • Created two failing testcases using buttons with CSS colors
• Clarifies how to compute the maximum possible contrast between two sets of colors.
• Add shadow to minimum contrast, similar to how it was applied to enhanced contrast

Notes:

• Please review issues (#1763 and #1791) to ensure that the widget changes match what was intended.
• @WilcoFiers @kengdoj please review drop shadow on minimum to ensure this is what was intended. I maintained the lower text contrast but added the same drop shadow as the enhanced version.

Closes issue(s):

• closes #1763
• closes #1791

Need for Call for Review: This will require a 2 weeks Call for Review << new rule, or substantial changes affecting a large number of test cases, if in doubt, use this.

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