Allow as_node_get_attribute_as_int() to parse negative numbers

This was failing as the gint64 value was being pushed into a guint64, so
negative numbers were being pushed above G_MAXINT and hence an error was being
returned.

Additionally, return G_MAXINT rather than G_MAXUINT, else we can't actually
parse the '-1' value.
Add <developer_name> from the draft AppStream 0.7 specification
Fix a strict validation failure when parsing .xml.in files

In this mode, treat translatable tags like <_name> as an indication the tag is
already translated.

Fixes: https://github.com/hughsie/appstream-glib/issues/2