Use NSApp abortModal instead of stopModal to end modal dialogs.

Use NSApp abortModal instead of stopModal to end modal dialogs.

Using abortModal is apparently the right thing to do if the dialog is not
being closed as the result of user action (but e.g. because a timer expired)
and shouldn't change anything if it done because of something the user did
(e.g. clicked on a button).

Closes #12467.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69949 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775