Fix build on X11, console and android.

src/dlangui/platforms/android/androidapp.d

@@ -33,6 +33,10 @@ class AndroidWindow : Window {
 	}
 	bool _visible;
 
+    override @property Window parentWindow() {
+        return null;
+    }
+    
 	protected dstring _caption;
 	/// returns window caption
 	override @property dstring windowCaption() {

src/dlangui/platforms/console/consoleapp.d

@@ -52,6 +52,11 @@ class ConsoleWindow : Window {
         Log.d("ConsoleWindow.close()");
         _platform.closeWindow(this);
     }
+    
+    override @property Window parentWindow() {
+        return _parent;
+    }
+
     protected bool _visible;
     /// returns true if window is shown
     @property bool visible() {

src/dlangui/platforms/x11/x11app.d

@@ -512,6 +512,10 @@ class X11Window : DWindow {
 		return result;
 	}
 
+    override @property DWindow parentWindow() {
+        return _parent;
+    }
+    
 	override @property dstring windowCaption() {
 		return _caption;
 	}