Add a new "Skip Unchanged" diff preference. When set to true, the Header
skips unchanged files when navigating to the previous or the next file.
Now that the side-by-side and the unified diff screens share the same
code base, this feature is made available in both of them.
Note that currently, the navigation links and keyboard shortcuts are not
updated upon toggling the option in PreferencesBox: they are only
updated when the preference is saved and the diff screen is reloaded.
To support updating without refreshing, we will need to replace the
KeyCommand in GlobalKey, which is of low priority and will be
implemented in a future change.
Bug: Issue 890
Change-Id: I0ee99be58a2be3d90983785ce03d218c2325d159
4da02542d1