Deduplicate default filename template code
Summary:
In preparation for user-visible improvements to the default filenames,
let's move the corresponding code around so that it only has to be
touched in a single place, preventing strings getting out of sync with
time.
Ref T9252
Test Plan:
Configure → Save dialog page should look like before, and
Save with a fresh config should populate the filename as usual.
Reviewers: Spectacle, ngraham
Reviewed By: Spectacle, ngraham
Subscribers: ngraham
Maniphest Tasks: T9252
Differential Revision: https://phabricator.kde.org/D14588