Improve opening the screenshots folder for missing config files

Summary:
If Save As was never used before and the Save button was
showing the Save As action, Spectacle would return Pictures by
default as the last used "Save As" location, while in all other cases it
would be Folder/. The missing / resulted in the filemanager
highlighting the folder only, instead of opening it.

The fix simply makes lastSaveAsLocation() consistently return URLs
with a / at the end.

Test Plan:

• rm ~/.config/spectaclerc
• Open Spectacle, Tools → Open Screenshots Folder
• Observe folder being opened instead of highlighted

Reviewers: Spectacle, gregormi, ngraham

Reviewed By: Spectacle, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D10559