`method.xml` is now able to specify a preferred position for each extra
keys in term of an other key to which it should be placed nearby.

It's implemented for French as an example.

The new PreferredPos class represents where an extra key should be
placed
Currently used to place keys at the same positions they were placed
before.

Don't replace dead-keys selected in the "Add keys to the keyboard"
option by an alternative.

The dead-key is replaced by its alternative if there's only one
specified.
Extra keys from every subtypes must be merged together to be able to
make this check.

For each extra key, a list of alternative can be specified. An extra key
won't be added to the keyboard if all its alternatives are already
present on it.

This is useful to avoid having the dead key for an accent and the
accented letters at the same time.

Allows to define a locale's script in 'method.xml' and use that to add
the extra keys for a locale to layouts of the same script only.

A locale of an undefined script will add its extra keys to every
layouts. A layout of an undefined script will have the extra keys of all
the enabled locales.