Implemented mouse leave event for GTK3 backend

src/Gtk/Avalonia.Gtk3/Interop/Native.cs

@@ -500,6 +500,24 @@ namespace Avalonia.Gtk3.Interop
         public gdouble delta_y;
     }
 
+    [StructLayout(LayoutKind.Sequential)]
+    unsafe  struct GdkEventCrossing 
+    {
+        public GdkEventType type;
+        public IntPtr window;
+        public gint8 send_event;
+        public IntPtr subwindow;
+        public guint32 time;
+        public gdouble x;
+        public gdouble y;
+        public gdouble x_root;
+        public gdouble y_root;
+        public int mode;
+        public int detail;
+        public bool focus;
+        public GdkModifierType state;
+    };
+    
     [StructLayout(LayoutKind.Sequential)]
     unsafe struct GdkEventWindowState
     {

src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs

@@ -45,6 +45,7 @@ namespace Avalonia.Gtk3
             ConnectEvent("window-state-event", OnStateChanged);
             ConnectEvent("key-press-event", OnKeyEvent);
             ConnectEvent("key-release-event", OnKeyEvent);
+            ConnectEvent("leave-notify-event", OnLeaveNotifyEvent);
             Connect<Native.D.signal_generic>("destroy", OnDestroy);
             Native.GtkWidgetRealize(gtkWidget);
             _lastSize = ClientSize;
@@ -194,6 +195,18 @@ namespace Avalonia.Gtk3
             return true;
         }
 
+        private unsafe bool OnLeaveNotifyEvent(IntPtr w, IntPtr pev, IntPtr userData)
+        {
+            var evnt = (GdkEventCrossing*) pev;
+            var position = new Point(evnt->x, evnt->y);
+            Input(new RawMouseEventArgs(Gtk3Platform.Mouse,
+                evnt->time,
+                _inputRoot,
+                RawMouseEventType.Move,
+                position, GetModifierKeys(evnt->state)));
+            return true;
+        }
+
         private unsafe bool OnCommit(IntPtr gtkwidget, IntPtr utf8string, IntPtr userdata)
         {
             Input(new RawTextInputEventArgs(Gtk3Platform.Keyboard, _lastKbdEvent, Utf8Buffer.StringFromPtr(utf8string)));