cmake-gui: Restore workaround for crash in file dialog
In commit ce9dbceb from !4836 (merged) 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 (closed)
Backport: release