Add themeVariant and color props to WebPartTitle

[mpowney]

Q	A
Bug fix?	[ ]
New feature?	[x]
New sample?	[ ]
Related issues?	fixes #258

What's in this Pull Request?

Two new properties have been added to the webPartTitle control:

• color
• themeVariant

Either can be used to override the colour of the web part title, to support section backgrounds.

When following the published guidance on enabling section backgrounds, the themeVariant property can be supplied to the webPartTitle component to apply the appropriate color.

Alternatively, the color property can be supplied to provide a direct override of the web part title's foreground color.

[codecov-io]

Codecov Report

Merging #316 into dev will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #316      +/-   ##
==========================================
+ Coverage   72.84%   72.88%   +0.03%     
==========================================
  Files          17       17              
  Lines         766      767       +1     
  Branches      148      149       +1     
==========================================
+ Hits          558      559       +1     
  Misses        161      161              
  Partials       47       47

Impacted Files	Coverage Δ
src/controls/webPartTitle/WebPartTitle.tsx	87.5% <100%> (+0.54%)	⬆️

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[estruyf]

@mpowney thank you very much for this! I'll merge this soon.

[estruyf]

Thanks @mpowney, this will be available in 1.14.0. I did a change to remove the color property, as this could be better achieved by specifying a custom class (which was already supported).