Add theme validator
This is on top of !93 (merged) and adds a validation tool for themes taking parent relationships into account. This makes it simple for users to build their own and also allows us to validate themes in feedbackd-device-themes.
It's a separate MR as I might tweak this more and I don't want to delay !93 (merged). Marking as draft until that is merged.
Edited by Guido Gunther