ChangeScreen2: Add mouse hover highlight to file table

Some users noted this table is wide and its hard to correlate file
path with size or number of comments when there are many files updated
in a single change.  Add a light blue background under the row the
mouse is hovering on, giving the user a bit more context to find all
related data cells.

Change-Id: I62c81d640c9e17926ccfd8827c89739a7632adba
2 files changed