[Toaster] Design: enhancements to variable history (5811)

[Barros Pena, Belen]

I've attached a design document to

https://bugzilla.yoctoproject.org/show_bug.cgi?id=5811

We would like to add the variable value and some source code to the
variable history information. The document describes how those could be
presented in Toaster.

Any feedback, suggestions, comments, etc let me know.

Thanks!

Belén