Attachment open confirmation dialogue: Show icons on buttons

The standard 'document-open" icon on the "Open With..." button, and the
application icon (from its service file) on the "Open with 'application'"
button.

This involves passing the service 'offer' to
AttachmentDialog::AttachmentDialog(), instead of the application name.

The dialogue text is converted to KUIT markup.

Differential Revision: https://phabricator.kde.org/D8162