tests: corrected bug in AdgDash offset checking
build: slightly simplified gir handling
adg: dropped ADG_TYPEDEF

This was a horrible hack to get gtk-doc working. Anyway it messed up the
GObject introspection side.

Dropped the hack and accessed a private struct in cairo.h (the lesser of
two evils). Rearranged code to still work with gtk-doc by using a helper
header file disabled only from the gtk-doc side.
doc: updated po
doc: updated italian translations
adg: added adg_dash_append_dashes_array()

Variable arguments and variadic containers are not easily bindable:
added an array version to let the bindings specify a dash pattern in one
go.
adg: improved AdgCanvas:scales handling

Changed private scales variable from GSList to a NULL terminated list of
strings and added PROP_SCALES property. Provided test coverage and
GObject introspection annotations.
adg: do not skip introspection of AdgPath methods
doc: updated italian translations