Disable text selection when dragging tooltip

[lucafalasco]

This css fix prevents code inside the pinned tooltip from getting highlighted when dragging it around.

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[codecov-io]

Codecov Report

Merging #264 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   20.57%   20.59%   +0.01%     
==========================================
  Files         291      291              
  Lines       11012    11012              
==========================================
+ Hits         2266     2268       +2     
+ Misses       8746     8744       -2

Impacted Files	Coverage Δ
...s/atom-ide-ui/pkg/atom-ide-definitions/lib/main.js	65.21% <0%> (+2.89%)	⬆️

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[hansonw]

Perfect! LGTM, thank you very much!