ChangeScreen2: Add preference to open side-by-side or unified diff view

A new user preference is added by which the user can decide which diff
screen (CodeMirror Side-by-Side or Unified Diff) is opened when
clicking on a file in ChangeScreen2. This preference doesn't apply to
the old change screen.

Bug: issue 2071
Change-Id: Ia1c7762c5559e383ad91ff0b8888c782b8134ef0
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
9 files changed