Respect user-specified filetype for Save As... operations

Summary:
Currently, when doing Save As... operations, Spectacle outright ignores the filetype you choose in Settings and always defaults to saving as a .png file. This patch fixes that.

BUG: 399773
FIXED-IN: 18.12.0

Test Plan:

1. Settings → Save → change the file type to something other than PNG
2. Save as...

The file dialog now comes pre-populated with the correct file type, and saving file saves it as that file type.

Tested with all available file types.

Reviewers: Spectacle, KDE Applications, broulik, utecht, alexeymin, shubham

Reviewed By: Spectacle, alexeymin

Subscribers: aacid

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D16190