Make 'Semantic Info' menu active (not sure why it was disabled)

[dpvc]

This PR enables the "Semantic Info" sub-menu in the "Explorer" menu. I'm not sure why it was disabled before. This fixes the attribute used for the "Prefix" selection, and adjusts the CSS for the tooltips so that the inner div has the same rounded corners as the outer container.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (f5a9b7f).
Report is 28 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1301   +/-   ##
========================================
  Coverage    86.71%   86.71%           
========================================
  Files          337      337           
  Lines        83979    83979           
  Branches      4750     4750           
========================================
  Hits         72826    72826           
- Misses       11130    11153   +23     
+ Partials        23        0   -23     

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[zorkow]

lgtm.