Herald v2 (#74)

* Add speech rate setting

* Add volume slider

* New speech manager skeleton

* User API key from resx

* Implement voice list retrieve via new api

* Rewrite to use ObAPI, remove all dependancies

* Use volume setting

* Clean up using statements

* Volume and timing adjustments

* Lookup rate value

* Use numeric rates for tighter spread

* Manage plugin data folder via core interface

* Add check that nullable settings are not null.

* Get file size before it's deleted.

* Improve old settings migration.

* Ignore cache sizes below 1MB

* Re-index orphaned files in cache, purge legacy wav files.

* Call top level error logging for native voice exception.

* Async title and detail requests to remove pause

* Remove NetCoreAudio use of temp files.

* Remove orphan using.

ObservatoryFramework/Interfaces.cs

@@ -194,5 +194,10 @@ namespace Observatory.Framework.Interfaces
         /// Returns true if the current LogMonitor state represents a batch-read mode.
         /// </summary>
         public bool IsLogMonitorBatchReading { get; }
+
+        /// <summary>
+        /// Retrieves and ensures creation of a location which can be used by the plugin to store persistent data.
+        /// </summary>
+        public string PluginStorageFolder { get; }
     }
 }