GVariant: improve bytestring support

 - add G_VARIANT_TYPE_BYTESTRING, _BYTESTRING_ARRAY, _STRING_ARRAY

 - remove g_variant_{new,get}_byte_array functions

 - add g_variant_{new,get,dup}_bytestring{,_array} functions

 - remove undocumented support for deserialising arrays of objectpaths
   or signature strngs using g_variant_get_strv()

 - add and document new format strings '^ay', '^&ay', '^aay' and '^a&ay'

 - update GApplication to use the new API

 - update GSettings binding code to use the new API

 - add tests