Remove the vuex.store content hack

[Daandelange]

The current implementation injects a kind-of "hack" in the vuex store to hide the content modifications from the notifications bar. The content changes are solely needed on the tabs-field to enable the when conditions within other fields.

By the way, it displays an warning/error on injection : [vuex] duplicate namespace content/ for the namespaced module content

For less hacky code and a more "kirby-native" integration with less risk of breaking itself, a different approach is needed, but I can't think of one.

Maybe there's something to do together with k3-whenquery, but that adds a dependency which might not really be needed ?
Otherwise, that repo might be a very useful inspiration for doing it properly anyways.