Calculate date and time values for file name at time of screen shot rather than time of save

Summary:
Added property to ExportManager to hold timestamp of screenshot that is then used when creating the file name instead of the file's save timestamp.

BUG: 394504
FIXED-IN: 18.12.0

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D15106