cmake-gui: Restore completion during path editing

Revert commit 0fc18188 (QCMakeWidgets: replace QDirModel, 2020-06-15,
v3.19.0-rc1~696^2).  `QFileSystemModel` is not a drop-in replacement for
`QDirModel`.  Additional changes are needed to make it work fully.
Revert the change and suppress the `QDirModel` deprecation warning
pending further investigation.

Fixes: #21589
14 jobs for cmake-gui-completion in 0 seconds (queued for 1 second)