better and faster window resize; Add "AllowsTransparency" to plugin interface; Use IPreviewHandler for Office files

QuickLook.Plugin/QuickLook.Plugin.IPreviewHandlers/PreviewPanel.xaml.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Runtime.InteropServices;
+using System.Windows.Controls;
+
+namespace QuickLook.Plugin.IPreviewHandlers
+{
+    /// <summary>
+    ///     Interaction logic for PreviewPanel.xaml
+    /// </summary>
+    public partial class PreviewPanel : UserControl, IDisposable
+    {
+        private PreviewHandlerHost _control = new PreviewHandlerHost();
+
+        public PreviewPanel()
+        {
+            InitializeComponent();
+        }
+
+        public void Dispose()
+        {
+            presenter.Child = null;
+            presenter?.Dispose();
+
+            _control?.Dispose();
+            _control = null;
+        }
+
+        public void PreviewFile(string file)
+        {
+            _control = new PreviewHandlerHost();
+
+            presenter.Child = _control;
+
+            _control.Open(file);
+
+            SetActiveWindow(presenter.Handle);
+        }
+
+        [DllImport("user32.dll")]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        private static extern bool SetActiveWindow(IntPtr hWnd);
+    }
+}