Make "Save As" the default save mode

Summary:
BUG: 389614

Yet more anecdotal evidence from bug triaging suggests that many would prefer "Save As" to be the default save mode in Spectacle. This patch accomplishes that.

I am planning a larger rewrite of the buttons sometime in the coming weeks that should hopefully end squabbles over button settings, but until then, I believe this will be a welcome improvement.

Test Plan:
Tested in KDE Neon
- Removed ~/.config/spectaclerc; "Save As" is now the default save mode
- Clicked "Save As" button: correct save operation is performed; button stays in "Save As" mode
- Changed the button to other save modes; same thing
- Quit Spectacle and re-opened when the button was in each save mode; it opened in the correct mode

{F5684691}

Reviewers: #spectacle, rkflx, rkron

Reviewed By: rkron

Differential Revision: https://phabricator.kde.org/D10198