add event overlay

[jpoehnelt]

supersedes #518

@ashleahhill I rebased this for you. only thing I need to look into is the impact on #531 and aria labels submitted by @crutchcorn.

closes #506

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[codecov]

Codecov Report

Merging #558 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #558   +/-   ##
=======================================
  Coverage   53.08%   53.08%           
=======================================
  Files          12       12           
  Lines         486      486           
  Branches       73       73           
=======================================
  Hits          258      258           
  Misses        228      228

Impacted Files	Coverage Δ
packages/markermanager/src/markermanager.ts	42.73% <0%> (ø)	⬆️
packages/markermanager/src/utils.ts	100% <0%> (ø)	⬆️
packages/markermanager/src/gridbounds.ts	50% <0%> (ø)	⬆️
packages/ogc/src/wmsmaptype.ts
packages/ogc/src/wmslayer.ts	100% <0%> (ø)

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[jpoehnelt]

@ashleahhill can you comment with @googlebot I consent. see #558 (comment)

[jpoehnelt]

@ashleahhill ping for cla approval