Don't delete wxTaskBarIcon in wxMSW wxNotificationMessage if we don't own it.

Don't delete wxTaskBarIcon in wxMSW wxNotificationMessage if we don't own it.

wxNotificationMessage can be associated with an existing wxTaskBarIcon under
wxMSW instead of creating its own one internally and in this case the icon
must not be deleted when the notification is hidden but it was always
destroyed unconditionally by wxNotificationIconEvtHandler created by
wxAutoNotifMsgImpl.

Fix this by only creating this helper event handler if we do own the icon.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69379 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775