feat(Dialog): add default aria-labelledby if not provided

[silviuaavram]

Fixes #812.

If Dialog has aria-label, do nothing.
If Dialog has aria-labelledbby, do nothing.
If Dialog does not have aria-label or aria-labelledby, get id from header prop and use it as aria-labelledby in Dialog.
If Dialog does not have aria-label or aria-labelledby, and header is string or simply has no id, generate id and add it as default prop to header and as aria-labelledby to Dialog.

[codecov]

Codecov Report

Merging #1298 into master will increase coverage by 0.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1298      +/-   ##
==========================================
+ Coverage   72.18%   72.55%   +0.37%     
==========================================
  Files         754      754              
  Lines        5622     5647      +25     
  Branches     1671     1653      -18     
==========================================
+ Hits         4058     4097      +39     
+ Misses       1557     1544      -13     
+ Partials        7        6       -1

Impacted Files	Coverage Δ
packages/react/src/components/Dialog/Dialog.tsx	36.11% <100%> (+36.11%)	⬆️
...ages/react/test/specs/behaviors/testDefinitions.ts	94.47% <100%> (+0.21%)	⬆️
...b/accessibility/Behaviors/Dialog/dialogBehavior.ts	100% <100%> (ø)	⬆️
packages/react/src/components/Portal/Portal.tsx	100% <0%> (+5%)	⬆️

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