Allow to stage smaller chunks from within DiffEditor

The Diff Editor already allows to stage/unstage individual chunks for commit.

However, especially on Windows, a "chunk" may be larger than one would expect (example: two changed lines with several unchanged lines in between are considered as one chunk).

From user side, two improvements looks reasonable:

1. A "chunk" should be each change delimited by one unchanged line. Creator should automatically split existing chunks If the underlying VCS has merged them together.
2. Even within a chunk, the possibility to stage individual lines should be given. This could be achived by dragging a selection over them.