SampleApp ValueHolder doesn't compare Values for Property Change Notification

[michael-hawker]

From discussion in #1399

There's a bug using the above though if the ValueHolder is changed, as it uses objects the != comparison here. I was pretty surprised, but it's because we're going through a object type conversion the enum value comparison isn't used unless we do if (!_value.Equals(value)) instead. See this article.

I also noticed we have some dead code in some of the samples which try and set their own datacontext when we do it for them now. All these sections can be removed. I'll file a bug to track these separately.

I'll submit a fix for these two things later this week.