[image wallpaper] Show wallpaper title/filename on grid view delegates

Summary:
This patch makes the wallpaper chooser grid view delegates show the wallpaper's
title (if it has a desktop file) or filename (if it doesn't). Authorship information
is shown via a tooltip, if present.

Test Plan:
{F6587535, size=full}
{F6587536, size=full}

Reviewers: #vdg, #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18737