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.
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.
This function is no longer an hardcoded list of layout ids. It's
replaced by a linear scan of the previously generated array and a new
corresponding array of resource ids.
`gen_layouts.py` lists the layouts in `res/xml` and generate the
`pref_layout_values` and `pref_layout_entries` arrays into
`res/values/layouts.xml`.
These arrays are hard to maintain as the order has to match, which is
fragile.
This relies on every layouts having a `name` attribute.
