Show the name of custom layouts in the list if it's provided using the
`name` attribute.
This should make managing several custom layouts easier.

This layout definition contains some documentation and is a better
default than no text.

Errors are obtained by running the parser, validation is throttle to
when the user stops editing for a second.

Line numbers will help reporting errors. Also, disable line breaking to
improve readability.

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.

This merges the "Layouts" option with the "Custom layout" option.
A custom layout becomes an item in the "Layouts" list among the other
layouts. It's possible to add several custom layouts.

Selecting the "Custom layout" item in the list opens a second dialog for
entering the layout description.

Layouts are serialized as JSON object and are decoded solely in the
LayoutsPreference class.