Update move/copy/link dialog window's title to reflect selected files

Summary:
When a single image has been selected, the window title now says, "Move/Copy/Link <filename>

When multiple image have been selected, the window title now says, "Move/Copy/Link <number of images selected> images"

Test Plan:
Tested in KDE Neon. Works fine.

Single image selection:

Multiple image selection:

Reviewers: rkflx, broulik, KDE Applications, ilic

Reviewed By: rkflx, ilic

Subscribers: ilic

Differential Revision: https://phabricator.kde.org/D8409