Allow saving screenshots in custom auto-generated subdirectories

Summary:
This patch allow the use of "/" in the filename field of the config dialog to save screenshots in auto-generated subdirectories inside the default save directory.

FEATURE: 394183

Test Plan: Tested for local and remote (Google Drive) save locations.

Reviewers: #spectacle, rkflx

Reviewed By: #spectacle, rkflx

Subscribers: ngraham, rkflx, #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D12993