Convert StandardLogfile loglevels properties to extension methods

[KristianVirkus]

Currently the StandardLogfile contains properties to easily and fluently get new log event instances by loglevel. Unfortunately, as soon as the StandardLogfile instance is handed over as ILogfileProxy<StandardLoglevel> these properties are no longer accessible.
These properties should be replaced in favor of extension methods on ILogfileProxy<StandardLoglevel> eventhough it's a slight loss of fluent syntax as paranthesis are required to get a new log event instance. Maybe it's even cleaner that way.
As an alternative, extension properties could be introduced as soon as they become available (see dotnet/roslyn#11159).