cmake-gui: Restore workaround for crash in file dialog

In commit ce9dbceb (QtDialog: remove Qt4-only code, 2020-06-02,
v3.19.0-rc1~712^2~1) we removed an event filter that was thought to be
necessary only as a workaround for a bug in Qt.  However, that bug was
fixed in Qt 4.5, and the file dialog still crashes as of Qt 5.14 without
the filter.  Restore the workaround pending further investigation.

Fixes: #21400