Preselect a sane default file name when asking for save-to location
Summary:
Previously, the user always had to write a file name from scratch.
Now the input file name is used by default. And because sometimes
that file is lacking a suffix (e.g. no .png for images), we also
try to auto-detect that.
Reviewers: dfaure, mlaurent
Reviewed By: dfaure, mlaurent
Differential Revision: https://phabricator.kde.org/D29460