Fix inability to Drag+Drop screenshot due to subfolder in filename

Summary:
Fixes the mentioned bug by creating all necessary subdirectories.

BUG: 417722

Test Plan:

1. Set save Filename in Spectacle to one including one or more /, thus including subdirectories. Example: %Y/%M/screen-%2d
2. Take a screenshot
3. Start dragging the preview

Reviewers: Spectacle, davidre, ngraham

Reviewed By: Spectacle, davidre, ngraham

Subscribers: ngraham, davidre, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D28702