Add support for TreeView decorators

[philschatz]

Allows a package to add/remove a class on the Directory or File similar to the FileIcon and the VCS updateStatus methods in tree-view.

FileIcon and the VCS updateStatus annotate the files/directories in different ways:

• FileIcon sets the class once based on the filename but does not allow other packages to change the class on their own.
• VCS methods in TreeView could be abstracted as a provider for decorating files and directories; most of the code so far in this Pull Request is copy/pasta from File::updateStatus.

Any thoughts on consolidating FileIcon and updateStatus methods by adding a decorator similar (but much simpler) to TextEditor::markBufferRange and TextEditor::decorateMarker?

This is a spike, mostly to see if there is interest before continuing

See the pull-requests package for the motivation for adding decorators:

[philschatz]

Any updates on this? I noticed other Pull Requests were given tags but maybe this one was skipped?

Also, there was a merge conflict in the tests but I rebased to remove the conflict. Thanks for making such a hackable editor!

[lee-dohm]

I've put it in the queue for review. I'm not sure when the team will be able to get to it, but it looks interesting!

[dac09]

I've not dived into writing atom plugins yet - so can't comment on the code, but going to request the atom team to have a look 👍