Add event-based LogMonitor state changes to better handle batch reads (#59)

* Add event-based LogMonitor state changes to better handle batch reads

Pre-reading hackily used read-all to suppress notifications. But that broke some assumptions about what read-all meant. Furthermore, the Core UI told plugins about read-all rather than the log monitor telling them -- which is really what should be telling them.

To address these concerns, LogMonitor now provides an event that both the PluginCore and PluginEventHandler listens to or tracking logging state allowing more granular information about the activities of LogMonitor, including distinguishing between ReadAll and Pre-read batches. Plugins no longer need to track if LogMonitor is in batch-read mode or not -- PluginCore now provides it. 

I've also converted all built-in plugins to use the new event-based system. The old system is marked deprecated and will go away once other known contributed plugins have converted to the new system.

* Change LogMonitorState enum to [Flags], drop 'None' state

As requested, and did associated simplifications and cleanup that followed.

ObservatoryFramework/EventArgs.cs

@@ -18,7 +18,7 @@ namespace Observatory.Framework
         /// </summary>
         public object journalEvent;
     }
-    
+
     /// <summary>
     /// Provides values used as arguments for Observatory notification events.
     /// </summary>
@@ -69,4 +69,40 @@ namespace Observatory.Framework
         PluginNotifier = 4,
         All = (NativeVisual | NativeVocal | PluginNotifier)
     }
+
+    [Flags]
+    public enum LogMonitorState : uint
+    {
+        // These need to be multiples of 2 as they're used via masking.
+        Idle = 0, // Monitoring is stopped
+        Realtime = 1, // Real-time monitoring is active
+        Batch = 2, // Performing a batch read of history
+        PreRead = 4 // Currently pre-reading current system context (a batch read)
+    }
+
+    /// <summary>
+    /// Provides information about a LogMonitor state transition.
+    /// </summary>
+    public class LogMonitorStateChangedEventArgs : EventArgs
+    {
+        /// <summary>
+        /// The previous LogMonitor state.
+        /// </summary>
+        public LogMonitorState PreviousState;
+
+        /// <summary>
+        /// The new, current LogMonitor state.
+        /// </summary>
+        public LogMonitorState NewState;
+
+        /// <summary>
+        /// Determins if the given state is a batch read of any form.
+        /// </summary>
+        /// <param name="state">The state to evaluate</param>
+        /// <returns>A boolean; True iff the state provided represents a batch-mode read.</returns>
+        public static bool IsBatchRead(LogMonitorState state)
+        {
+            return (state & (LogMonitorState.Batch | LogMonitorState.PreRead)) > 0;
+        }
+    }
 }