Add autosave feature

Summary:
FEATURE: 390415
Add an option to automatically save the screenshot immediately after it has been taken

Test Plan:

1. Enable the option Autosave the image to 'Save location' with 'Filename'
2. Take a screenshot
3. Image gets saved ASAP

If you launch Spectacle with a hotkey (for example: PrtSc), it takes the screenshot and saves the image immediately.

Reviewers: VDG, Spectacle, davidre, ngraham

Reviewed By: VDG, Spectacle, davidre, ngraham

Subscribers: davidre, ngraham, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D23210