accessibility fixes

[rlskoeser]

• update document title when leaf is selected #263
• keep disabled leaves disabled when tree is zoomed #265
• visually dim labels when leaves are disabled #264

[codecov[bot]]

Codecov Report

Merging #274 (5dba834) into develop (ba25caf) will increase coverage by 0.20%. The diff coverage is 33.33%.

:exclamation: Current head 5dba834 differs from pull request most recent head a053fcc. Consider uploading reports for the commit a053fcc to get more accurate results

@@             Coverage Diff             @@
##           develop     #274      +/-   ##
===========================================
+ Coverage    15.81%   16.02%   +0.20%     
===========================================
  Files            9        9              
  Lines          746      749       +3     
  Branches       146      147       +1     
===========================================
+ Hits           118      120       +2     
  Misses         544      544              
- Partials        84       85       +1