Don't save to a temp file if exporting to another app

Summary:
When exporting a screenshot to a local app for further processing, the image is saved to /tmp/, which causes issues in the other app, not least of which is that the file is not writable, so Save operations fail.

This patch makes Spectacle save the file to the default location when exporting to another app.

BUG: 399395
FIXED-IN: 18.12.0

Test Plan:

1. Export → [some app or Other Application...]
2. Observe that the screenshot is saved at the default location, and then opened in the app of your choice
3. Edit the image in that app and do File → Save
4. Observe that it works!

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

Reviewed By: Spectacle, alexeymin

Subscribers: pino

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D16191