SideBySide2: Color file header when in full screen mode

When the file header is "scrolled off" the view and the panels go
nearly full screen the file header is still visible at the top of the
file.  Currently there is a single 1px grey bar that separates the
file header from the rest of the content.  This can be a bit too
subtle and the top of the file just sort of "disappears".

When the panels go "full screen" add the same grey as the line number
gutters to the header bar, making it look more like a header.

Change-Id: Ic76ea320a1dd2bfe449e262d1d9e39ffbcecfa77
1 file changed