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.
The script `check_layout.py` checks some properties about layouts.
No check is an error.
The result of running this script on every layouts is stored in the file
`check_layout.output`, which is useful to track changes.
Add make rules to run this script as well as `sync_translations`.
