Previously, we would always (1) put English first, and (2) sort the
other layouts based on their English description. This fixes both
issues.
* gnu/installer/newt/keymap.scm (sort-layouts)[layout<?]: New procedure.
[preferred]: New variable.
Partition according to both the 'name' and 'synopsis' fields. Sort both
the main layouts and the other layouts according to 'layout<?'.