Add a new parameter `--landscape-layers` as well as an environment
variable `WVKBD_LANDSCAPE_LAYERS` that allows the user to override the
default set of layers in landscape mode. This complements the existing
`-l` parameter and `WVKBD_LAYERS` environment variable, which do the
equivalent for non-landscape mode.
Furthermore, add documentation for the new switch. Move the `-l`
parameter to the bottom so we don't have to realign all parameters and
so all layer-related parameters are grouped together.