Add loading api functions for all plugins

Author: Jack251970

Flow.Launcher.Core/Plugin/JsonRPCV2Models/JsonRPCPublicAPI.cs

@@ -175,5 +175,15 @@ public void BackToQueryResults()
         {
             _api.BackToQueryResults();
         }
+
+        public void StartLoadingBar()
+        {
+            _api.StartLoadingBar();
+        }
+
+        public void StopLoadingBar()
+        {
+            _api.StopLoadingBar();
+        }
     }
 }

Flow.Launcher.Plugin/Interfaces/IPublicAPI.cs

@@ -335,5 +335,15 @@ public interface IPublicAPI
         /// <param name="forceClosed">When user closes the progress box manually by button or esc key, this action will be called.</param>
         /// <returns>A progress box interface.</returns>
         public Task ShowProgressBoxAsync(string caption, Func<Action<double>, Task> reportProgressAsync, Action forceClosed = null);
+
+        /// <summary>
+        /// Start the loading bar in main window
+        /// </summary>
+        public void StartLoadingBar();
+
+        /// <summary>
+        /// Stop the loading bar in main window
+        /// </summary>
+        public void StopLoadingBar();
     }
 }