Preserve colour of tab-icons

[Alhadis]

Description of the Change

This theme currently overrides the colour of tab-icons to make them more visible when viewed against a tinted background. As @silvestreh explains, "if you have a green icon with a green background you wouldn't see it."

Although well-intentioned, this solution breaks two user expectations:

• The enableVcsColoring setting of the core tabs package is ignored. It's likely (and logical) that a tab's icon share the colour applied to its label. When VCS-aware tabs are enabled, the label's colour is rendered in orange or green for changed and added files, respectively. However, icons are singled out, making it look very inconsistent:

• The file-icons package can't display coloured tab-icons. This is difficult to fix with user stylesheets, because file-icons uses many different CSS classes to apply colour. The solution is essentially copy+pasting the package's colour classes en masse and affixing !important; to each property. Needless to say, that's neither elegant, practical, or future-proof:

  @import "packages/file-icons/styles/colours";
  
  // Copy-pasted from above stylesheet, with !important appended.
  .light-red:before     { color: @light-red !important; }
  .medium-red:before    { color: @medium-red !important; } // … etc

  Yet the opposite is trivial to achieve:

  // Show uncoloured icons in the tab-bar pane only
  .tab-bar .tab .title.icon::before{
      color: inherit; // Note there's no need to include an !important flag
  }

Caveats / Possible Drawbacks

• ce3ea0c will only take effect once file-icons/atom@1dfac6c lands in the package's next release. Once this is merged, I'll get one cut within a few days.
• Some users may prefer the existing behaviour. It can be easily restored using the above snippet, though.

Applicable Issues

• Fixes file-icons/atom#652
• Fixes atom-material/atom-material-ui#438
• Fixes atom-material/atom-material-ui#349

[silvestreh]

Thanks for the time you put into the PR description! I'll merge it now 👍