Make linenumbers relative to file

[SebDieBln]

This fixes #67. Comments to lines of a file are now referenced to the actual line in the file, not the line in the diff-view. If the comment is for a deleted line, the line-number within the old file is used and saved along with a minus-sign as a flag to indicate it is for the old file.

There is room for further improvement:

• Migrate of existing comments using default values for creating the diff-view.
• Show an indicator if a comment is hidden, because the line it is on is outside the current diff-view.