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.
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.
The two layout selection options are replaced by a ListGroupPreference
that allow to enter an arbitrary amount of layouts.
The "switch_second" and "switch_second_back" keys are replaced by
"switch_forward" and "switch_backward", which allow to cycle through the
selected layouts in two directions.
Layouts are changed to place these two key on the space bar.
The backward key is not shown if there's only two layouts.
