Avoid crashes when creating initially hidden MDI child under Unity.

Avoid crashes when creating initially hidden MDI child under Unity.

Ignore "hide" signals for the menus without associated shown window. Ubuntu
Unity sends them and we crashed because of a recursive assert in the
corresponding signal handler before.

Now the code doesn't crash any more but the menus still don't behave correctly
when the last MDI child is destroyed.

Closes #13593.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72379 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775