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