Popup: Add z-index to popper container

[felixmosh]

fixes #4083

[codecov-commenter]

Codecov Report

Merging #4084 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4084   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         180      180           
  Lines        3231     3231           
=======================================
  Hits         3223     3223           
  Misses          8        8           

Impacted Files	Coverage Δ
src/modules/Popup/Popup.js	98.76% <ø> (ø)

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[siarheipashkevich]

please merge this PR

[layershifter]

@felixmosh thanks for contributing, however the solution with syncing zIndex from .popup is more flexible for users than a hardcoded value. I implemented it #4094 and we plan to release it on Monday. Will appreciate your eyes there 🙏

[felixmosh]

Closing since #4094 fixes the same bug