Adds gr-diff-cursor, which is a specialized cursor for navigating
through diff content. It tracks the diff line being targeted, and, in
side-by-side mode, it tracks the side of the diff being targeted.
Also includes special behavior that ships blank spaces and other
non-commentable content in diffs, as well as automatically switching
sides when appropriate.
Support for more than one diff.
Also updates the diff builders to emit some useful element classes.
Note that this only adds the diff cursor along with tests. This is
setting up the work to actually add the diff cursor to the diff views.
Bug: Issue 4033
Change-Id: I991fd514f56ddd19d43d8e1354ad0d4fc71930c4
