Refactor saveMode handling to use enums instead of ints

Summary:
Things like case 0 make it really hard to work with the code and
having access to the current state of the Save button is needed
for a subsequent patch.

Test Plan: Spectacle still remembers the last used save state, defaults work too.

Reviewers: Spectacle, ngraham

Reviewed By: Spectacle, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D10467