[Toaster] Changes to the Toaster navigation (7329)

[Barros Pena, Belen]

Based on the outcome of the usability testing sessions we ran in March, an
in order to accommodate the image customisation feature we will be adding
in 1.9, the Toaster navigation system needs a bit of revamp.

The design document attached to

https://bugzilla.yoctoproject.org/show_bug.cgi?id=7329

shows the required changes.

If you have any questions or feedback, please let me know.

Thanks!

Belén