Added scrollMode property to ScrollView to simplify Rulers

Added scrollMode property to ScrollView to simplify Rulers

The rulers are hard to use and understand, and require overriding
classes in the public API. This makes binding to C# require too much
effort, and pulls too many classes into C# that otherwise don't need to
be there.

Have instead added a devel property to enable the simplification of
setting up rulers - the new property map will be used internally to
create either DefaultRuler or FixedRuler, depending on the
settings. This makes it very easy to bind to C#.

Change-Id: I5932cd32487bae77b8c41ebd35e35f6bc879be33