Adds filterable viewer

[StephenDemanuele]

Added a usercontrol which uses the same datacontext properties, but the end result is different on these counts:

1. Has a collection of checkboxes which allow the user to toggle logging levels
2. Uses DataGrid instead of ListView. We can implement filtering within the grid later on. A usecase would be to lookout for events based on a keyword in the FormattedMessage field.
3. DataGrid scrolls to keep the last added item in view.

So current filtering is applied on the log level. Updated Sample to use the new usercontrol.