Add autosave feature

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

Test Plan:
{F7252033}

  # Enable the option //Autosave the image to 'Save location' with 'Filename'//
  # Take a screenshot
  # 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