Change "Open..." to "Add Image..." in Wallpaper chooser

Summary:
Changes the button in the wallpaper chooser marked "Open..." to say "Add Image..". This matches the text style for the button in slideshow mode, which says, "Add Folder"

Also added ellipses to "Add Folder"; it's now "Add Folder..."

Test Plan:
Tested in KDE Neon.

Before:

After:

Reviewers: Plasma, broulik, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: aspotashev, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8658