Speed up exit time

Speed up exit time

Instantiating a single QSettings and using it for each ui element
can significantly shorten the time required to save the various
states at exit. The speed up can be better appreciated on *nix,
where the settings are saved on disk, rather than on Windows where
they are held in memory (in the registry).

(cherry picked from commit 3b43fbfbb18cc4535aba3fd77c6a3cf21835dafa)