fixes #212

refs #127

This increases the chance of a perfect 3 column layout.

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

Although the docs [1] specify a class on Popover components, this
doesn’t seem to be passed down. We help ourselves with a selector for
the data-headlessui-state attribute .

[1] https://headlessui.com/vue/popover

* deactivate buttons and show progress indicator while saving
* place primary button first
* add spacing between buttons
* add support for custom labels for cancel and save

The FormGroup’s save-indicator would jump around when used in
combination with the FormTable. We now have a more tight coupling for
these components so that the FormTable can influence the FormGroup’s
rendering.

fixes #199

The update method can be used to update more than one field for which
errors might be displayed in separate locations.

Feedback hasn’t been to good, so we remove these again.

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