Add cssValue condition.

[rosolko]

Close #628 issue.

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request.
If it fixes a bug or resolves a feature request, be sure to link to that issue.

Checklist

• Checkstyle and unit tests pass locally with my changes by running gradle check chrome htmlunit command
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Coverage decreased (-0.1%) to 66.745% when pulling 26d5bef on rosolko:cssValue into 8276676 on codeborne:master.

[codecov-io]

Codecov Report

Merging #727 into master will decrease coverage by 0.12%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #727      +/-   ##
============================================
- Coverage     63.56%   63.43%   -0.13%     
+ Complexity      866      865       -1     
============================================
  Files           154      154              
  Lines          2983     2989       +6     
  Branches        291      291              
============================================
  Hits           1896     1896              
- Misses          989      994       +5     
- Partials         98       99       +1

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/com/codeborne/selenide/Condition.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...e/selenide/impl/WebDriverThreadLocalContainer.java	81.61% <0%> (-0.74%)	30% <0%> (-1%)
...ava/com/codeborne/selenide/ElementsCollection.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
.../codeborne/selenide/impl/SelenideElementProxy.java	86.79% <0%> (+0.25%)	26% <0%> (ø)	⬇️

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[vinogradoff]

Could you please extend your integration test for one computed css property example?
means element doesn't have explicit "style" attribute, but the css property is set due to class/css rules etc.

[rosolko]

@vinogradoff done.