Add annotation level information to gutter tooltip

[akoreman]

Currently, the gutter tooltip only shows the annotation text without any indication whether the messages are of the level 'error', 'warning' or 'info'. See image for current state of the gutter tooltip:

This change improves this by adding a header to the gutter tooltip indicating the total number of annotations per level for that row and an icon in front of each message indicating the annotation level for that annotation. Image of the gutter tooltip after this change:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Patch coverage: 99.12% and project coverage change: +0.15 🎉

Comparison is base (9f86bb0) 86.62% compared to head (8fb72a1) 86.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5101      +/-   ##
==========================================
+ Coverage   86.62%   86.78%   +0.15%     
==========================================
  Files         555      556       +1     
  Lines       42996    43097     +101     
  Branches     6709     6714       +5     
==========================================
+ Hits        37247    37400     +153     
+ Misses       5749     5697      -52     

Flag	Coverage Δ
unittests	86.78% <99.12%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/css/editor.css.js	100.00% <ø> (ø)
src/mouse/default_gutter_handler_test.js	98.83% <98.83%> (ø)
src/layer/gutter.js	90.49% <100.00%> (+0.03%)	⬆️
src/mouse/default_gutter_handler.js	64.95% <100.00%> (+39.71%)	⬆️

... and 1 file with indirect coverage changes

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[nightwing]

Looks good, and good to merge, but one thing that we should address in a follow-up pr, is that we should try to remove all the uses of innerHTML and use the safer buildDom method. This would help to remove lang.escapeHTML from setAnnotations for instance and keep annotation objects instead of breaking them into arrays of text and type.