[WPF] Fixed drag operation starting even when mouse is outside TreeView

This closes #752

Xwt.WPF/Xwt.WPFBackend/ExTreeViewItem.cs

@@ -133,6 +133,7 @@ protected override void OnMouseLeftButtonDown (MouseButtonEventArgs e)
 		{
 			if (!view.SelectedItems.Contains (this.DataContext) || CtrlPressed)
 				view.SelectItem (this);
+			view.Backend.WidgetMouseDownForDragHandler (this, e);
 			e.Handled = true;
 			base.OnMouseLeftButtonDown (e);
 		}
@@ -141,6 +142,7 @@ protected override void OnMouseLeftButtonUp (MouseButtonEventArgs e)
 		{
 			if (view.SelectedItems.Contains (this.DataContext) && !CtrlPressed)
 				view.SelectItem (this);
+			view.Backend.WidgetMouseUpForDragHandler(this, e);
 			e.Handled = true;
 			base.OnMouseLeftButtonUp (e);
 		}

Xwt.WPF/Xwt.WPFBackend/WidgetBackend.cs

@@ -645,6 +645,11 @@ void WidgetMouseDownHandler (object o, MouseButtonEventArgs e)
 				e.Handled = true;
 		}
 
+		internal void WidgetMouseDownForDragHandler(object o, MouseButtonEventArgs e)
+		{
+			SetupDragRect(e);
+		}
+
 		void WidgetMouseUpHandler (object o, MouseButtonEventArgs e)
 		{
 			var args = e.ToXwtButtonArgs (Widget);
@@ -742,6 +747,7 @@ public void SetDragSource (TransferDataType [] types, DragDropAction dragAction)
 			DragDropInfo.TargetTypes = types == null ? new TransferDataType [0] : types;
 			Widget.MouseUp += WidgetMouseUpForDragHandler;
 			Widget.MouseMove += WidgetMouseMoveForDragHandler;
+			Widget.MouseDown += WidgetMouseDownForDragHandler;
 		}
 
 		private void SetupDragRect (MouseEventArgs e)
@@ -752,7 +758,7 @@ private void SetupDragRect (MouseEventArgs e)
 			DragDropInfo.DragRect = new Rect (loc.X - width / 2, loc.Y - height / 2, width, height);
 		}
 
-		void WidgetMouseUpForDragHandler (object o, EventArgs e)
+		internal void WidgetMouseUpForDragHandler (object o, EventArgs e)
 		{
 			DragDropInfo.DragRect = Rect.Empty;
 		}
@@ -765,7 +771,7 @@ void WidgetMouseMoveForDragHandler (object o, MouseEventArgs e)
 				return;
 
 			if (DragDropInfo.DragRect.IsEmpty)
-				SetupDragRect (e);
+				return;
 
 			if (DragDropInfo.DragRect.Contains (e.GetPosition (Widget)))
 				return;