show comments and drafts count in changes browser

- for each file in the changes browser
- made ReviewCommentSink observable (to be notified of new or
  deleted/removed comments)

Closes: #80
add PathUtils class

- move getRelativePath to new utils class (multiple usages)
Merge pull request #86 from forrert/changes-browser/file-comment-count

show comments and drafts count in changes browser