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.
`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.
