SideBySide2: Display red bar in line number gutter at padding

Instead of the bare gutter where padding widgets are inserted, display
a red bar in the line number gutter so the reader can visually follow
that this side of the file is the prior line number connected to the
next line number, and isn't just a rendering glitch.

Change-Id: I9792fb6a93a15c25f5bd31da2f478c2c28ab8f48
3 files changed