We often find ourselves adding flex styles to buttons. Instead, this
should be the default.

This refactors the existing state helpers (useCopy and
useUpdatableState) and unifies them to useUpdateBehaviour, useCopy,
useAPIObjectFieldCopy, useRelation and useRelationList. This eliminates
a lot of custom code in the show settings page and unifies the behaviour
of all input fields and save operations making the experience more
consistent and understandable.

It also enables us to show the save-in-progress state and errors that
occurred during updates for all fields along with customizable debounce
and proper v-model support.

We also got rid of the remaining uses of useSelectedShow. The show
settings page still made use of it despite using a route that
provides the relevant show that should be edited.

refs #185

refs #185

API object attribute names are now:

1. camelCased (#164)
2. use Id / Ids suffix for object references (#172)

Image uploads might take a while and even though we can’t currently show
a progress bar for the upload (because fetch upload progress is still
not a thing) we can at least indicate that something is happening and
disable any interactive UI elements.

refs #144

isProcessing seems more general purpose as it’s not clear if the
provided function actually loads something.

Using :is-open="true" interferes with the dialogs close function in
nested dialogs.

Using the same name as the native HTML dialog is a bad idea.

closes #139