SideBySide2: Add user preference to hide top menu

The top menu occupies a lot of space. I personally prefer the
version of SideBySide2 that hides the top menu when CM3 has scrolled
down some number of lines. Allow users to hide the top menu as a
preference, gaining back some screen space for code.

Change-Id: Ib30e8875fcc4b87662a5c12fa0bf3cc791d02c95
11 files changed