Don't prevent app exit when only preferences window exists.

Don't prevent termination of the app if all windows are closed, except
for the preferences one (which may very well be hidden at the moment).
This only affects platforms where the preferences are modeless (GTK+, OS
X) and where the preferences window, once created, continues to exist,
but is hidden.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77521 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775