gapi.xsd: Fix type of array_len attribute in fieldType
generator: Add support for fixed length arrays as method parameters

Fixed length arrays are available in gobject introspection and are
already converted by the bindinator tool. The array_len attribute was
only used in structs.

This adds support for them as method parameters, generating the correct
code for them. Fixes issue #98.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>