added placeholder implementations for system dialogs.

src/Android/Perspex.Android/SystemDialogImpl.cs

@@ -12,5 +12,10 @@ namespace Perspex.Android
         {
             throw new NotImplementedException();
         }
+
+        public Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
+        {
+            throw new NotImplementedException();
+        }
     }
 }

src/Gtk/Perspex.Gtk/SystemDialogImpl.cs

@@ -53,5 +53,10 @@ namespace Perspex.Gtk
             dlg.Show();
             return tcs.Task;
         }
+
+        public Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
+        {
+            throw new NotImplementedException();
+        }
     }
 }