Improve readability of saving glue code

Summary:
Instead of two calls to `doSave*` with different boolean values, we can
simply pass the bool directly. This makes the code easier to understand.

Test Plan:
{nav Save} and {nav Save As} should work as before with any state of
{nav Quit after Save or Copy}.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D13244