Remember the last-used save mode by default

Summary: Anecdotal evidence from bug triaging, internet forums, and reviews of KDE software suggests that 99% of Spectacle's users are unaware that this setting exists, and many would like its behavior turned on by default. This patch accomplishes that.

Test Plan:
Tested in NDE Neon:
- Removed ~/.config/spectaclerc
- Opened Spectacle and confirmed that the button remembers the last-used save mode by default and that the applicable setting is toggled by default

Reviewers: #spectacle, broulik, rkron, kossebau, egorov, rkflx

Reviewed By: rkron, rkflx

Subscribers: bgupta, rkflx

Differential Revision: https://phabricator.kde.org/D10153