The new `layouts` preference replaces three previous preferences:

    layout
    second_layout
    custom_layout

Add a preference migration function, which first migration is to
migrate layouts into the new preference.

The migration must also be called from the SettingsActivity as it might
use a different preference store due to the boot-aware preference copy.

Remove the cross button on the right of custom layouts to reduce
accidental removal.
A remove button is added in the dialog for modifying the layout.

Clicking on a custom layout opens a dialog for modifying the layout
description instead of the dialog for selecting a layout.

[Preference.onClick] is apparently not called on [Item]s, though it is
on [AddButton].

Workaround this by listening on click events on the view.

(De)serializing might raise exceptions, which are handled by dropping
the failing item.

Allow items to be of any class instead of strings.

Item serialization and deserialization methods are in a separate class
because they are also used in a static context.