Add showPopover method, update example

[minikin]

• Add showPopover method
• Update example

[codecov]

Codecov Report

Merging #18 (d133df9) into main (065673d) will increase coverage by 0.26%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   78.57%   78.83%   +0.26%     
==========================================
  Files          12       12              
  Lines         462      463       +1     
==========================================
+ Hits          363      365       +2     
+ Misses         99       98       -1     

Impacted Files	Coverage Δ
lib/src/popover.dart	38.77% <36.36%> (+3.35%)	⬆️

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