Give Gwenview's Save As dialog a proper title: "Save Image"

Summary:
BUG: 348221

Gives Gwenview's Save As dialog a proper title: "Save Image"

Test Plan:
Tested in KDE Neon (dev unstable). Works as expected:

Reviewers: KDE Applications, aacid, cfeck, broulik

Reviewed By: aacid, cfeck

Differential Revision: https://phabricator.kde.org/D7746