[Toaster] Design - Editing package content in custom images

[Barros Pena, Belen]

I've documented how we add and remove packages to / from custom images,
and opened this enhancement

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8117

Design attached to the Bugzilla entry, as usual. The only tricky thing is
the dependency handling, but we used to this in Hob, so I am assuming we
can do this in Toaster as well.

Any questions / comments, let me know.

Thanks,

Belén