notificationDaemon: only display a large image if an icon is also specified

notificationDaemon: only display a large image if an icon is also specified

Historically, when applications set "image-data" they expect it to show up
as an icon. So we display it as such if an icon is not specified with an
"app_icon" argument to Notify(). We also use "image-path" for an icon if
an icon is not specified.

We only display a large image specified with "image-data" or "image-path"
if an icon is also specified.

https://bugzilla.gnome.org/show_bug.cgi?id=659158