refs #206

fixes #203

fixes #210

fixes #209

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 #127

refs #127

refs #182

refs #191

refs #181

refs #181

Until now the show context menu was only displayed when user’s were
visiting routes for that particular show. As we keep a global show
selection state, we can display this context menu at any time, because
the state is not unset when users visit other dashboard sites.

At least for now that makes sense.

This component will help us to outline areas in the dashboard app, that
are scheduled for major overhauls. This should make feedback more
effective as people testing the dashboard can focus their time & effort
on areas that haven’t received the love they deserve.