Feature: Support Pixbuf Icon on the FDO Notifcation backend

[lovetox]

Currently if a application sets a Pixbuf as icon on a Gio.Notification object, it depends on if the org.gtk.Notification is available or not if the icon makes it through. (org.gtk.Notification supports pixbuf icons)

The FDO backend simply ignores a pixbuf and does not pass it on.

It looks trivial from the code to support Pixbuf because its exactly what the G_IS_BYTES_ICON does, minus loading the bytes into a pixbuf.

[jsparber]

Libnotify does this already https://gitlab.gnome.org/GNOME/libnotify/-/blob/master/libnotify/notification.c#L1353

I won't have time now to implement it though.