Improve MVVM support for some Toolkit controls

[nCastle1]

Search and FloorFilter both depend on setting a DataContext in the constructor. This doesn't work if there is an externally-set DataContext, which is quite common in MVVM scenarios.

The quick fix is to add a wrapping FrameworkElement in the control template, and explicitly set the DataContext to {x:Null}, then set the DataContext on the child.

[dotMorten]

Custom controls should not set the datacontext anywhere, but instead use TemplateBinding.