fix(Portal): fix several event related behaviors

[levithomason]

Fixes #1271 Do not steal focus on portal close
Fixes #1251 Do not stop propagation of events
Fixes #1076 Popups in Modals open in the correct position

This PR also:

• fixes the mountNode=null bug in for the Modal
• adds Modal prop closeOnDimmerClick
• keeps a Portal open when blurring the trigger and focusing the Portal

[codecov-io]

Codecov Report

Merging #1273 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master    #1273      +/-   ##
==========================================
+ Coverage   95.91%   95.92%   +<.01%     
==========================================
  Files         882      883       +1     
  Lines        4916     4927      +11     
==========================================
+ Hits         4715     4726      +11     
  Misses        201      201

Impacted Files	Coverage Δ
...mples/modules/Dimmer/States/DimmerExampleActive.js	100% <ø> (ø)	✅
...modules/Dimmer/Variations/DimmerExampleBlurring.js	50% <ø> (ø)	✅
...amples/modules/Dimmer/Usage/DimmerExampleLoader.js	50% <ø> (ø)	✅
...Dimmer/Variations/DimmerExampleBlurringInverted.js	50% <ø> (ø)	✅
...modules/Dimmer/Variations/DimmerExampleInverted.js	50% <ø> (ø)	✅
...mples/modules/Dimmer/Types/DimmerExampleContent.js	50% <ø> (ø)	✅
...amples/modules/Dimmer/Types/DimmerExampleDimmer.js	50% <ø> (ø)	✅
src/elements/Label/Label.js	100% <ø> (ø)	✅
...amples/modules/Dimmer/Usage/DimmerExampleEvents.js	60% <ø> (ø)	✅
...s/modules/Dimmer/Variations/DimmerExampleSimple.js	100% <ø> (ø)	✅
... and 5 more

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[levithomason]

Released in semantic-ui-react@0.64.7.