qt: make control bar model behave like a value type again

Addressable types that can not behave like a value type, such as having an external "ID" field, bring considerable problems and hinder maintenance. It becomes harder to serialize, which is a necessity since we want to save the toolbar layout, and new tricky questions arise such as: are two models that have exactly the same layout and elements the "same", even if their IDs differ?

This partially reverts commit 09ca278c, and uses deep comparison instead of associating ID to the model for selection purposes.

Request review @chub.